Preparation of catalytically active multielement oxide materials which contain at least one of the elements Nb and W and the elements Mo, V and Cu

ABSTRACT

Catalytically active multielement oxide materials which contain at least one of the elements Nb and W and the elements Mo, V and Cu are prepared by a process in which an intimate dry blend containing ammonium ions is prepared and said dry blend is subjected to a thermal treatment in an atmosphere having a low molecular oxygen content at elevated temperatures, a portion of the ammonium ions contained in the intimate dry blend being decomposed with liberation of ammonia and the oxygen content of the thermal treatment atmosphere being increased in the course of the thermal treatment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and incorporates by reference thefollowing applications: U.S. Provisional Application No. 60/530,617,filed on Dec. 19, 2003; U.S. Provisional Application No. 60/475,488,filed on Jun. 4, 2003; German Patent Application No. 103 25 488.9, filedon Jun. 4, 2003, and German Patent Application 103 60 058.2, filed onDec. 19, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the preparation ofcatalytically active multielement oxide materials which contain at leastone of the elements Nb, W and the elements Mo, V and Cu, the molarfraction of the element Mo, based on the total amount of all elements ofthe catalytically active multielement oxide material, other than oxygen,being from 20 to 80 mol %, the molar ratio of Mo contained in thecatalytically active multielement oxide material to V, Mo/V contained inthe catalytically active multielement oxide material being from 15:1 to1:1, the corresponding molar ratio Mo/Cu being from 30:1 to 1:3 and thecorresponding molar ratio Mo/(total amount of W and Nb) being 80:1 to1:4, in which an intimate dry blend also containing ammonium ions isprepared from starting compounds which contain the elementalconstituents of the multielement oxide material, other than oxygen, ascomponents and said dry blend is thermally treated in an atmospherehaving a low content of molecular oxygen at elevated temperatures, atleast a portion of the ammonium ions contained in the intimate dry blendbeing decomposed at ≧160° C. with liberation of ammonia.

The present invention also relates to a process for the preparation ofacrylic acid by heterogeneously catalyzed partial gas-phase oxidation ofacrolein using catalysts which contain the abovementioned multielementoxide materials as catalytically active materials.

The process, described at the outset, for the preparation ofcatalytically active multielement oxide materials is known, as is theuse of the multielement oxide materials obtainable thereby as activematerial in catalysts for the heterogeneously catalyzed partialgas-phase oxidation of acrolein to acrylic acid.

2. Discussion of the Background

DE-31 19 586 C2 discloses that a catalytically active multielement oxidematerial which contains the elements Mo and V as base component can beprepared by preparing an intimate dry blend comprising ammonium ionsfrom starting compounds which contain the elemental constituents or themultielement oxide materials as components, and thermally treating saiddry blend at 380° C. in a gas stream which contains 1% by volume ofmolecular oxygen.

The resulting multielement oxides are recommended as active material forcatalysts for the catalytic partial gas-phase oxidation of acrolein toacrylic acid.

DATABASE WPI, Week 7512, Derwent Publication Ltd., London, UK; AN75-20002 & JP-A 49097793 (Asahi Chemical Ind. Co.), Sep. 19, 1974,recommends the thermal treatment of corresponding intimate dry blendsfor the preparation of relevant multielement oxide active materials withcompete exclusion of molecular oxygen. EP-A 113 156 recommends carryingout the thermal treatment in an air stream. EP-A 724481 states that thethermal treatment should be carried out in such a way that the contentof molecular oxygen at any point during the thermal treatment in the(gaseous) treatment atmosphere is from 0.5 to 4% by volume. In theexemplary embodiment, the molecular oxygen content in the thermaltreatment atmosphere was 1.5% by volume.

In the exemplary embodiments of EP-A 714700, the thermal treatment ofthe intimate dry blend is carried out both in an air stream and in anatmosphere whose molecular oxygen content was 1.5% by volume.

DE-A 10046928, DE-A 19815281 and EP-A 668104 show that multielementoxide active materials which are mentioned at the outset and have amultiphase structure are particularly suitable as active materials forcatalysts for the heterogeneously catalyzed partial oxidation ofacrolein to acrylic acid when at least one phase is separately preformedfor the preparation of the intimate dry blend to be thermally treatedand the thermal treatment is carried out in a gas atmosphere whichcontinuously contains from 1.5 to 1.4% by volume of molecular oxygen.

A disadvantage of the prior art is that substantially all of itrecommends a thermal treatment of the intimate dry blend at a content ofmolecular oxygen which is substantially constant over the duration ofthe thermal treatment in the associated gas atmosphere.

However, relevant multielement oxide active materials obtained in thismanner are not completely satisfactory with regard to activity andselectivity when used as active materials in catalysts for theheterogeneously catalyzed partial oxidation of acrolein to acrylic acid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a rotary tube furnace.

FIG. 2 shows the percentage of M^(A) as a function of the materialtemperature in ° C.

FIG. 3 shows the ammonia concentration of the atmosphere A in % byvolume over the thermal treatment as a function of the materialtemperature in ° C.

FIG. 4 shows the pore distribution of the milled active material powderbefore its shaping (its specific surface area was 21 m²/g). The porediameter in μm is plotted along the abscissa (logarithmic scale).

FIG. 5 shows the individual contributions of the individual porediameters (abscissa, in Angstrom, logarithmic scale) in the microporeregion to the total pore volume for the active material powder beforeits shaping, in ml/g (ordinate).

FIG. 6 shows the same as FIG. 4, but for multimetal oxide activematerial (its specific surface area was 24.8 m²/g) subsequently detachedfrom the annular coated catalyst by scratching off mechanically.

FIG. 7 shows the same as FIG. 5, but for multimetal oxide activematerial subsequently detached from the annular coated catalyst byscratching off mechanically.

FIG. 8 shows the analog of FIG. 6 (the specific surface area of thescratched-off multimetal oxide active material was 20.3 m²/g).

FIG. 9 shows the analog of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is an object of the present invention to provide an improved processfor the preparation of relevant multielement oxide active materials,which gives multielement oxide active materials which, when used asactive materials in catalysts for the heterogeneously catalyzed partialoxidation of acrolein to acrylic acid, have high activity and highselectivity of the acrylic acid formation.

We have found that this object is achieved by a process for thepreparation of catalytically active multielement oxide materials whichcontain at least one of the elements Nb and W and the elements Mo, V andCu, the molar fraction of the element Mo, based on the total amount ofall elements other than oxygen in the catalytically active multielementoxide material, being from 20 (preferably 30 or 40) to 80 mol %, themolar ratio of Mo contained in the catalytically active multielementoxide material to V, Mo/V contained in the catalytically activemultielement oxide material being from 15:1 to 1:1, the correspondingmolar ratio Mo/Cu being from 30:1 to 1:3 and the corresponding molarratio Mo/(total amount of W and Nb) being from 80:1 to 1:4, in which anintimate dry blend also containing ammonium ions is prepared fromstarting compounds which contain the elemental constituents of themultielement oxide material, other than oxygen, as components and saiddry blend is thermally treated at elevated temperatures in an (gas)atmosphere having a low content of molecular oxygen, at least a portionof the ammonium ions contained in the intimate dry blend beingdecomposed at ≧160° C. with liberation of ammonia, wherein the thermaltreatment is carried out as follows:

-   -   the intimate dry blend is heated at a heating rate of ≦10°        C./min to a decomposition temperature in the decomposition        temperature range from 240° C. to 360° C. and is kept in this        temperature range until at least 90 mol % of the total amount        M^(A) of ammonia liberated altogether in the entire course of        the thermal treatment of the intimate dry blend from the        intimate dry blend at above 160° C. have been liberated;    -   the content of molecular oxygen in the (gas) atmosphere A in        which the thermal treatment of the intimate dry blend takes        place is reduced to ≦0.5% by volume no later than when the        intimate dry blend has reached 230° C., and this low oxygen        content is maintained until at least 20, preferably at least 30,        particularly preferably at least 40, mol % of the total amount        M^(A) of ammonia liberated altogether in the entire course of        the thermal treatment have been liberated;    -   the intimate blend is taken at a rate of ≦10° C./min out of the        decomposition temperature range and into the calcination        temperature range of from 380 to 450° C. no earlier than when        ≧70 mol % (frequently when ≧75 mol % or when ≧80 mol % or ≧85        mol % or ≧90 mol %) of the total amount of M^(A) of ammonia        liberated altogether in the entire course of the thermal        treatment have been liberated and    -   the content of molecular oxygen in the atmosphere A is increased        to >0.5 to 4% by volume no later than when 98 mol % or 95 mol %        of the total amount M^(A) of ammonia liberated altogether in the        entire course of the thermal treatment have been liberated        and the intimate dry blend is calcined at this increased oxygen        content of the atmosphere A in the calcination temperature        range.

Of course, in the novel process, the elements Mo, V, Cu and Nb and/or W(like all other elements other than oxygen which, if desired, arepresent) are contained in oxidic (and not in metallic, elemental) formin the catalytically active multielement oxide material obtainableaccording to the invention.

The content of ammonium ions in the intimate dry blend to be thermallytreated according to the invention is advantageously at least 5 or atleast 10, preferably at least 20, particularly preferably at least 30,very particularly preferably at least 40, mol %, based on the totalmolar content of elemental constituents of the subsequent catalyticallyactive multielement oxide material, other than oxygen, in the intimatedry blend. As a rule, the ammonium content, on the above basis, of theintimate dry blend, is ≦150 mol % or ≦100 mol %, in general ≦90 mol % or≦80 mol %, frequently ≦70 mol % or ≦60 mol %.

In the novel process, the temperature rate at which the intimate dryblend is heated to the decomposition temperature is advantageously ≦8,preferably ≦5, particularly preferably ≦3, very particularly preferably≦2 or ≦1° C./min. As a rule, however, this temperature rate is ≧0.1, ingeneral ≧0.2, ° C./min, frequently ≧0.3 or ≧0.4° C./min.

The abovementioned also applies to the temperature rate at which theintimate dry blend is brought out of the decomposition range and intothe calcination range of from 380 to 480° C.

According to the invention, the decomposition temperature range ispreferably from 280 to 360° C., particularly preferably from 300 to 350°C. or from 310 to 340° C.

In the novel process, the intimate dry blend is furthermoreadvantageously kept in the decomposition temperature range until atleast 95, preferably at least 97, more preferably at least 99, mol % orall of the total amount M^(A) of ammonia liberated altogether in theentire course of the thermal treatment of the intimate dry blend have orhas been liberated.

In the novel process, the intimate dry blend is usually heated from roomtemperature (e.g. 20 or 25 or 30 or 35 or 40° C.) to the decompositiontemperature.

In the novel process, the content of molecular oxygen in the (gas)atmosphere A in which the thermal treatment of the intimate dry blendtakes place must be reduced to ≦0.5% by volume no later than when theintimate dry blend has reached 230° C.

It is preferable according to the invention if the content of molecularoxygen is reduced to ≦0.3, particularly preferably to ≦0.1, % by volume.Particularly preferably, the content of molecular oxygen in theatmosphere A is insignificant in this phase of the novel process. As arule, however, this oxygen content is ≧0.05% by volume.

In the novel process, the content of the atmosphere A is preferably ≦0.5or ≦0.3 or ≦0.1 or 0% by volume even below 230° C. (e.g. even attemperatures ≧200° C.). As a rule, however, this oxygen content is≧0.05% by volume even below 230° C. (e.g. even at temperatures <200°C.).

Below 230° C. or 200° C., however, the gas atmosphere A in which thethermal treatment takes place may also have substantially higher oxygencontents. Below 230° C. or 200° C. in the novel process, this content ofmolecular oxygen can in principle be ≧5 or ≧10 or ≧15 or ≧20 or ≧25 or30% by volume or more. The thermal treatment atmosphere comprisingsubstantially exclusively air or molecular oxygen is also possible inthis temperature range.

According to the invention, the content of ≦0.5% by volume of molecularoxygen in the atmosphere A is maintained at least until at least 20 or30 or 40 mol %, preferably at least 50 mol %, particularly preferably atleast 60 mol % or at least 70 mol % or at least 80 mol %, of the totalamount M^(A) of ammonia liberated altogether in the entire course of thethermal treatment have been liberated.

Experience has shown that it is advantageous if the content of molecularoxygen in the atmosphere A is, however, increased to >0.5 to 4% byvolume even before 95 mol % of the total amount M^(A) of ammonialiberated altogether in the entire course of the thermal treatment havebeen liberated. This increase in the oxygen content is preferablyeffected even before 90, preferably before 0.85, mol % and particularlypreferably no later than when 80 mol % of the total amount M^(A) ofammonia liberated altogether in the entire course of the thermaltreatment have been liberated.

This means that it is expedient according to the invention if the rangeof the novel process in which the content of molecular oxygen in theatmosphere A is ≦0.5% by volume extends until 20 or 30 or 40 or 80 mol%, preferably from 50 to 70 mol %, of the total amount M^(A) of ammonialiberated altogether in the entire course of the thermal treatment havebeen liberated.

If the content of molecular oxygen in the atmosphere A is increased to≦0.5 to 4% by volume no later than when 98 mol % or 95 mol % of thetotal amount M^(A) of ammonia liberated altogether in the entire courseof the thermal treatment have been liberated, this increase ispreferably effected to a value of ≧0.55 to 4, particularly preferably≧0.6 to 4, % by volume. Very particularly preferably the increase in theoxygen content is effected to a value of from 1 to 3 or from 1 to 2% byvolume. These values relating to increases also apply to increases inthe case of all other liberated portions of the total amounts of ammonialiberated altogether, which portions are mentioned as being possible.

In the novel process, the calcination temperature range advantageouslycovers a temperature of the intimate dry blend of from 380 to 430° C.,particularly preferably from 390 to 420° C.

In the novel process, the decomposition temperature range is thattemperature range after which the decomposition of the ammonium ionscontained in the intimate dry blend to be thermally treated according tothe invention is substantially complete.

In the novel process, the formation of the catalytically activemultielement oxide takes place in the calcination temperature range.

As a rule, the calcination in the calcination temperature range willlast for at least 10, preferably at least 20, particularly preferably atleast 30, minutes. As a rule, the calcination in the calcinationtemperature range continues for ≦2 hours, frequently ≦1.5 hours or ≦1hour.

After calcination is complete, the calined material is usually cooled.As a rule, it is cooled to room temperature (e.g. to 20° C. or to 25° C.or to 30° C. or to 35° C. or to 40° C.).

It is expedient according to the invention if the calcined material iscooled to ≦100° C. within a period of ≦5, preferably ≦4, particularlypreferably ≦3 or ≦2, hours. As a rule, however, this cooling period isnot less than 0.5 hour.

Advantageously, the cooling of the calcined material is effected in a(gas) atmosphere A which surrounds it and whose content of molecularoxygen is ≦5 or ≦4 or ≦3 or ≦2 or ≦1 or ≦0.5% by volume, preferably ≦0.3or ≦0.1 or 0% by volume, as a rule ≧0.05% by volume. This oxygen contentis expediently established at the start of bringing the calcinedmaterial, still present in the calcination temperature range, out of thecalcination temperature range by reducing the temperature.

Once the calcined material has cooled to ≦350° C. or ≦300° C. or ≦250°C., the further cooling can also be effected in a (gas) atmosphere Awhose content of molecular oxygen is ≧5 or ≧10 or ≧15 or ≧20 or ≧25 or≧30% by volume or more.

A (gas) atmosphere A comprising substantially exclusively air ormolecular oxygen is also possible during the further cooling below thistemperature.

In addition to the described contents of molecular oxygen, theatmosphere A in which the thermal treatment of the intimate dry blendtakes place is substantially composed of the components escaping ingaseous form from the intimate dry blend and of inert gas. The terminert gases is understood as meaning all those gases which do notchemically react with the dry material to be thermally treated accordingto the invention. Examples of inert gases are N₂ or noble gases. Theatmosphere A will contain steam in particular when the intimate dryblend contains water of hydration. As a rule, the steam content of theatmosphere A does not exceed 20% by volume at any time during the novelthermal treatment. As a rule, it is even ≦10% by volume at all times.

In the novel process, the ammonia content of the (gas) atmosphere Ausually passes through a maximum which is usually ≦10, frequently ≦8, %by volume and generally ≦7% by volume. Usually, however, it is above 1,frequently above 2 or 3, % by volume.

The ammonia content of the atmosphere A usually passes through itsmaximum before the intimate dry blend has reached the calcinationtemperature range.

This means that, in the calcination temperature range, the maximumammonia content of the atmosphere A is as a rule ≦2 or ≦1% by volume.However, it is usually >0% by volume.

(Arithmetically) averaged over the total time during which the intimatedry blend is in the calcination temperature range, the NH₃ content ofthe atmosphere A is as a rule ≦1, preferably ≦0.5, % by volume.(Arithmetically) averaged over the total time during which the intimatedry blend is in the temperature range >160° C. and ≦360° C., the NH₃content of the atmosphere A is usually 1 or 1.5 or from 2 to 8% byvolume, in general from 1 to 4% by volume.

In the novel process, usually no external ammonia is added to the (gas)atmosphere A, i.e. the only ammonia source usually comprises theammonium ions incorporated into the intimate dry blend. However, it maybe expedient in the novel process to remove (gas) atmosphere Acontinuously and to recycle it to a certain extent (i.e. to recycle itto the intimate dry blend to be thermally treated).

According to the invention, it is advantageous to stop the calcinationbefore MoO₃ is detectable in the X-ray diffraction pattern. However,MoO₃ contents of up to 30 or up to 20% by weight are tolerable in themultielement oxide active materials obtainable according to theinvention.

In addition to the elements Nb and/or W, and Mo, V and Cu, themultielement oxide active materials obtainable according to theinvention may additionally contain, for example, the elements Ta, Cr,Ce, Ni, Co, Fe, Mn, Zn, Sb, Bi, alkali metal (Li, Na, K, Rb, Cs), H,alkaline earth metal (Mg, Ca, Sr, Ba), Si, Al, Ti and Zr. According tothe invention, however, the multielement oxide active material can ofcourse also consist only of the elements Nb and/or W and Mo, V and Cu.

Catalytically active multielement oxide materials obtainable accordingto the invention and particularly suitable as active material forcatalysts for the heterogeneously catalyzed partial gas-phase oxidationof acrolein to acrylic acid (and of methacrolein to methacrylic acid andof propane to acrylic acid; the novel products of the process are alsosuitable for these heterogeneously catalyzed gas-phase partialoxidations) satisfy the following stoichiometry IMo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)  (I)where:

-   X¹ is W, Nb, Ta, Cr and/or Ce,-   X² is Cu, Ni, Co, Fe, Mn and/or Zn,-   X³ is Sb and/or Bi,-   X⁴ is one or more alkali metals (Li, Na, K, Rb, Cs) and/or H,-   X⁵ is one or more alkaline earth metals (Mg, Ca, Sr, Ba),-   X⁶ is Si, Al, Ti and/or Zr,-   a is from 1 to 6,-   b is from 0.2 to 4,-   c is from 0.5 to 18,-   d is from 0 to 40,-   e is from 0 to 2,-   f is from 0 to 4,-   g is from 0 to 40 and-   n is a number which is determined by the valency and frequency of    the elements other than oxygen in I, and    in which the variables are to be chosen within the specified ranges    with the proviso that the molar fraction of the element Mo, based on    the total amount of all elements other than oxygen in the    multielement oxide material (I), is from 20 to 80 mol %, the molar    ratio of Mo contained in the catalytically active multielement oxide    material (I) to V, Mo/V, contained in the catalytically active    multielement oxide material (I) is from 15:1 to 1:1, the    corresponding molar ratio Mo/Cu is from 30:1 to 1:3 and the    corresponding molar ratio Mo/(total amount of W and Nb) is from 80:1    to 1:4.

Preferred among the active multielement oxide materials (I) are those inwhich the variables are in the following ranges:

-   X¹ is W, Nb and/or Cr,-   X² is Cu, Ni, Co and/or Fe,-   X³ is Sb,-   X⁴ is Na and/or K,-   X⁵ is Ca, Sr and/or Ba,-   X⁶ is Si, Al and/or Ti,-   a is from 2.5 to 5,-   b is from 0.5 to 2,-   c is from 0.5 to 3,-   d is from 0 to 2,-   e is from 0 to 0.2,-   f is from 0 to 1,-   g is from 0 to 15 and-   n is a number which is determined by the valency and frequency of    the elements other than oxygen in I.

Very particularly preferably, however, the following multielement oxideactive materials II are direct products of the novel process:Mo₁₂V_(a)X¹ _(b)X² _(c)X⁵ _(f)X⁶ _(g)O_(n)  (II),where:

-   X¹ is W and/or Nb,-   X² is Cu and/or Ni,-   X⁵ is Co and/or Sr,-   X⁶ Si and/or Al,-   a is from 3 to 4.5,-   b is from 1 to 1.5,-   c is from 0.75 to 2.5,-   f is from 0 to 0.5,-   g is from 0 to 8 and-   n is a number which is determined by the valency and frequency of    the elements other than oxygen in II, and    in which the variables are to be chosen within the specified ranges    with the proviso that the molar fraction of the element Mo, based on    the total amount of all elements other than oxygen in the    multielement oxide active material (II), is from 20 to 80 mol %, the    molar ratio of Mo contained in the catalytically active multielement    oxide material (II) to V, Mo/V, contained in the catalytically    active multielement oxide material (II) is from 15:1 to 1:1, the    corresponding molar ratio Mo/Cu is from 30:1 to 1:3 and the    corresponding molar ratio Mo/(total amount of W and Nb) is from 80:1    to 1:4.

In the novel process, sources (starting compounds) of the elementalconstituents of the desired multielement oxide active material, otherthan oxygen, in the respective stoichiometric ratio desired in themultielement oxide active material, are used as starting materials forthe preparation of such novel direct products of the process, saidsources being suitable in a manner known per se, and a very intimate,preferably finely divided, dry blend is produced from said sources andis then subjected to the novel thermal treatment, it being possible tocarry out the thermal treatment before or after the shaping to catalystmoldings of a certain geometry. According to the invention, saidtreatment is advantageously carried out beforehand. The sources can beeither oxides or those compounds which can be converted into oxides byheating, at least in the presence of oxygen. In addition to the oxides,particularly suitable starting compounds are therefore halides,nitrates, formates, oxalates, acetates, carbonates and hydroxides.

Suitable starting compounds of Mo, V, W and Nb are also oxo compoundsthereof (molybdates, vanadates, tungstates and niobates) or the acidsderived from these. Oxygen-containing sources are advantageous for thenovel process.

The content of ammonium ions which is required according to theinvention in the intimate dry blend can be realized in a simple mannerby incorporating a corresponding amount of ammonium ions into theintimate dry blend. The ammonium ions can be expediently introduced intothe intimate dry blend, for example, by using, as sources of theelements Mo, V, W or Nb, the corresponding ammonium oxometallates.Examples of these are ammonium metaniobate, ammonium metavanadate,ammonium heptamolybdate tetrahydrate and ammonium paratungstateheptahydrate. However, ammonium donors, such as NH₄NO₃ or NH₄Cl orammonium acetate or ammonium carbonate or ammonium bicarbonate or NH₄OHor NH₄CHO₂ or ammonium oxalate, can of course also be incorporated intothe intimate dry blend to be thermally treated, independently of thestarting compounds required as sources of the multielement oxide activematerial constituents.

The thorough mixing of the starting compounds can be carried out inprinciple in dry or wet form. If it is effected in dry form, thestarting compounds are expediently used in the form of finely dividedpowder and, after the mixing, compressed (e.g. tableted) to givecatalyst moldings of the desired geometry, which are then subjected tothe novel thermal treatment.

However, the thorough mixing is preferably effected in wet form.Usually, the starting compounds are mixed with one another in the formof an aqueous solution and/or suspension. Particularly intimate dryblends are obtained in the mixing method described when exclusivelysources and starting compounds present in dissolved form are employed. Apreferably used solvent is water. Thereafter, the aqueous material(solution or suspension) is dried and the intimate dry blend thusobtained is, if required, subjected directly to the thermal treatmentaccording to the invention. The drying process is preferably effected by

(the outlet temperatures are as a rule from 100 to 150° C.) andimmediately after the preparation of the aqueous solution or suspension.The resulting powder can be directly molded by compression. Frequently,however, it proves to be too finely divided for direct furtherprocessing and is then therefore expediently kneaded with addition of,for example, water. The addition of a lower organic carboxylic acid(e.g. acetic acid) often proves advantageous during kneading (typicaladded amounts are from 5 to 10% by weight, based on powder materialused).

The resulting kneaded material is then either shaped to give the desiredcatalyst geometry, dried and then subjected to the novel thermaltreatment (leads to unsupported catalysts) or is calcined in theunmolded form and then milled to give a powder (usually <80 μm,preferably <50 μm, particularly preferably <30 μm, as a rule ≧1 μm),which is usually applied as moist material to inert supports withaddition of a small amount of water and, if required, furtherconventional binders. After the end of the coating, drying is carriedout again and a ready-to-use coated catalyst is thus obtained. If thethorough mixing of the starting compounds is effected in the form of,for example, an aqueous solution, inert porous supports may also beimpregnated with said solution, dried and then subjected to the thermaltreatment according to the invention to give supported catalysts. In thepreparation of coated catalysts, the coating of the supports can also becarried out before the novel thermal treatment, for example with themoistened spray-dried powder.

Support materials suitable for coated catalysts are, for example, porousor nonporous aluminas, silica, thorium dioxide, zirconium dioxide,silicon carbide or silicates, such as magnesium silicate or aluminumsilicate (e.g. steatite of the type C 220 from CeramTec).

The supports may have a regular or irregular shape, those having aregular shape and pronounced surface roughness, e.g. spheres or hollowcylinders coated with chips, being preferred.

The use of substantially nonporous, spherical steatite supports whichhave a rough surface (e.g. steatite of the type C 220 from CeramTec) andwhose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, ispreferred. However, the use of cylinders as supports whose length isfrom 2 to 10 mm and whose external diameter is from 4 to 10 mm is alsosuitable. In the case of annular supports, the wall thickness ismoreover usually from 1 to 4 mm. Annular supports preferably to be usedhave a length of from 2 to 6 mm, an external diameter of from 4 to 8 mmand a wall thickness of from 1 to 2 mm. Rings measuring 7 mm×3 mm×4 mm(external diameter×length×internal diameter) are also particularlysuitable as supports.

The coating of the supports with finely divided multielement oxideactive material obtainable according to the invention or with the finelydivided precursor material thereof (intimate dry blend) to be subjectedto the thermal treatment according to the invention is carried out as arule in a rotatable container, as disclosed, for example, in DE-A2909671, EP-A 293859 or EP-A 714700. The procedure of EP-A 714700 ispreferred.

The support is expediently moistened for coating the supports with thepowder material to be applied. After the application, drying is usuallyeffected by means of hot air. The coat thickness of the powder materialapplied to the support is expediently chosen to be in the range of from10 to 1 000 μm, preferably from 50 to 500 μm, particularly preferablyfrom 150 to 250 μm.

In the case of unsupported catalysts, the shaping can, as stated above,also be effected before or after the novel thermal treatment is carriedout.

For example, unsupported catalysts can be prepared from the powder formof the multielement oxide active material obtainable according to theinvention or its precursor material not as yet subjected to the thermaltreatment (the intimate dry blend), by compaction to give the desiredcatalyst geometry (e.g. by tableting or extrusion), it being possible,if required, to add assistants, e.g. graphite or stearic acid aslubricants and/or molding assistants and/or reinforcing agents, such asmicrofibers of glass, asbestos, silicon carbide or potassium titanate.Suitable geometries for unsupported catalysts are, for example, solidcylinders or hollow cylinders having an external diameter and a lengthof from 2 to 10 mm. In the case of the hollow cylinders, a wallthickness of from 1 to 3 mm is expedient. The unsupported catalyst canof course also have a spherical geometry, it being possible for thesphere diameter to be from 2 to 10 mm.

The multielement oxide active materials obtainable according to theinvention can of course also be used in powder form, i.e. withoutshaping to give certain catalyst geometries, as catalysts for theheterogeneously catalyzed partial oxidation of acrolein to acrylic acidor methacrolein to methacrylic acid or propane to acrylic acid (forexample also in a fluidized bed).

However, the novel process is also suitable for the preparation ofmultielement oxide active materials of the formula III[A]_(p)[B]_(q)[C]_(r)  (III),where:

-   A is Mo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x),-   B is X₁ ⁷Cu_(h)H_(i)O_(y),-   C is X₁ ⁸Sb_(j)H_(k)O_(z),-   X¹ is W, Nb, Ta, Cr and/or Ce, preferably W, Nb and/or Cr,-   X² is Cu, Ni, Co, Fe, Mn and/or Zn, preferably Cu, Ni, Co and/or Fe,-   X³ is Sb and/or Bi, preferably Sb,-   X⁴ is Li, Na, K, Rb, Cs and/or H, preferably Na and/or K,-   X⁵ is Mg, Ca, Sr and/or Ba, preferably Ca, Sr and/or Ba,-   X⁶ is Si, Al, Ti and/or Zr, preferably Si, Al and/or Ti,-   X⁷ is Mo, W, V, Nb and/or Ta, preferably Mo and/or W,-   X⁸ is Cu, Ni, Zn, Co, Fe, Cd, Mn, Mg, Ca, Sr and/or Ba, preferably    Cu and/or Zn, particularly preferably Cu,-   a is from 1 to 8, preferably from 2 to 6,-   b is from 0.2 to 5, preferably 0.5 to 2.5,-   c is from 0 to 23, preferably from 0 to 4,-   d is from 0 to 50, preferably from 0 to 3,-   e is from 0 to 2, preferably from 0 to 0.3,-   f is from 0 to 5, preferably from 0 to 2,-   g is from 0 to 50, preferably from 0 to 20,-   h is from 0.3 to 2.5, preferably from 0.5 to 2, particularly    preferably from 0.75 to 1.5,-   i is from 0 to 2, preferably from 0 to 1,-   j is from 0.1 to 50, preferably from 0.2 to 20, particularly    preferably from 0.2 to 5,-   k is from 0 to 50, preferably from 0 to 20, particularly preferably    from 0 to 12,-   x, y and z are numbers which are determined by the valency and    frequency of the elements other than oxygen in A, B and C,-   p and q are positive numbers,-   r is 0 or a positive number, preferably a positive number, the ratio    p/(q+r) being from 20:1 to 1:20, preferably from 5:1 to 1:14,    particularly preferably from 2:1 to 1:8, and, where r is a positive    number, the ratio q/r being from 20:1 to 1:20, preferably from 4:1    to 1:4, particularly preferably from 2:1 to 1:2, very particularly    preferably 1:1,    which contain the moiety [A]_(p) in the form of three-dimensional    regions (phases) A having the chemical composition    A: Mo₁₂VaX¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x),    the moiety [B]_(q) in the form of three-dimensional regions (phases)    B having the chemical composition    B: X₁ ⁷Cu_(h)H_(i)O_(y) and    the moiety [C]_(r) in the form of three-dimensional regions (phases)    C having the chemical composition    C: X₁ ⁸Sb_(j)H_(k)O_(z),    the regions A, B and, if required, C being distributed relative to    one another as in a mixture of finely divided A, finely divided B    and, if required, finely divided C, and    in which all variables are to be chosen within the specified ranges    with the proviso that the molar fraction of the element Mo, based on    the total amount of all elements other than oxygen in the    multielement oxide active material (III), is from 20 to 80 mol %,    the molar ratio of the Mo contained in the catalytically active    multielement oxide material (III) to V, Mo/V, contained in the    catalytically active multielement oxide material (III) is from 15:1    to 1:1, the corresponding molar ratio of Mo/Cu is from 30:1 to 1:3    and the corresponding molar ratio Mo/(total amount of W and Nb) is    from 80:1 to 1:4.

Preferred multielement oxide active materials II are those whose rangesA have a composition in the following stoichiometric range of theformula IVMo₁₂V_(a)X¹ _(b)X² _(c)X⁵ _(f)X⁶ _(g)O_(x)  (IV),where

-   X¹ is W and/or Nb,-   X² is Cu and/or Ni,-   X⁵ is Ca and/or Sr,-   X⁶ is Si and/or Al,-   a is from 2 to 6,-   b is from 1 to 2,-   c is from 1 to 3,-   f is from 0 to 0.75,-   g is from 0 to 10 and-   x is a number which is determined by the valency and frequency of    the elements other than oxygen in (IV).

The term phase used in connection with the multielement oxide activematerials III means three-dimensional regions whose chemical compositiondiffers from that of their environment. The phases are not necessarilyX-ray homogeneous. As a rule, the phase A is a continuous phase in whichparticles of the phase B and, if required, C are dispersed.

The finely divided phases B and, if required, C advantageously consistof particles whose diameter, i.e. the longest distance passing throughthe center of gravity of the particle and connecting two points presenton the surface of the particle, is up to 300 μm, preferably from 0.1 to200 μm, particularly preferably from 0.5 to 50 μm, very particularlypreferably from 1 to 30 μm. However, particles having a diameter of from10 to 80 μm or from 75 to 125 μm are also suitable.

In principle, the phases A, B and, if required, C in the multimetaloxide active materials III obtainable according to the invention may bepresent in amorphous and/or crystalline form.

It is advantageous if the phase B consists of crystallites ofoxometallates or contains such oxometallate crystallites (=oxidecrystallites) which have the X-ray diffraction pattern, and hence thecrystal structure type, of at least one of the following coppermolybdates. The source of the associated X-ray diffraction fingerprintis shown in brackets.

-   Cu₄Mo₆O₂₀ [A. Moini et al., Inorg. Chem. 25 (21) (1986) 3782-3785],-   Cu₄Mo₅O₁₇ [Index card 39-181 of the JCPDS-ICDD Index (1991)],-   α-CuMoO₄ [Index card 22-242 of the JCPDS-ICDD Index (1991)],-   Cu₆Mo₅O₁₈ [Index card 40-865 of the JCPDS-ICDD Index (1991)],-   Cu_(4-x)Mo₃O₁₂ where X is from 0 to 0.25 [Index card 24-56 and    26-547 of the JCPDS-ICDD Index (1991)],-   Cu₆Mo₄O₁₅ [Index card 35-17 of the JCPDS-ICDD Index (1991)],-   Cu₃(MoO₄)₂(OH)₂ [Index card 36-405 of the JCPDS-ICDD Index (1991)],-   Cu₃Mo₂O₉ [Index card 24-55 and 34-637 of the JCPDS-ICDD Index    (1991)],-   Cu₂MoO₅ [Index card 22-607 of the JCPDS-ICDD Index (1991)].

The phase B preferably contains oxometallates which have the X-raydiffraction pattern, and hence the crystal structure type, of thefollowing copper molybdate:

-   CuMoO₄-III having the wolframite structure according to Russian    Journal of Inorganic Chemistry 36 (7) (1991), 927-928, table 1.

Preferred among these are those having the following stoichiometry VCu₁Mo_(A)W_(B)V_(C)Nb_(D)Ta_(E)O_(y).(H₂O)_(F)  (V),where

-   1/(A+B+C+D+E) is from 0.7 to 1.3, preferably from 0.85 to 1.15,    particularly preferably from 0.95 to 1.05, very particularly    preferably 1,-   F is from 0 to 1,-   B+C+D+E is from 0 to 1, preferably from 0 to 0.7, and-   Y is a number which is determined by the valency and frequency of    the elements other than oxygen.

Particularly preferred among these are those having the stoichiometryVI, VII or VIII:Cu₁Mo_(A)W_(B)V_(C)O_(y)  (VI),where

-   1/(A+B+C) is from 0.7 to 1.3, preferably from 0.85 to 1.15,    particularly preferably from 0.95 to 1.05, very particularly    preferably 1,-   B+C is from 0 to 1, preferably from 0 to 0.7, and-   y is a number which is determined by the valency and frequency of    the elements other than oxygen;    Cu₁Mo_(A)W_(B)O_(y)  (VI),    where-   1/(A+B) is from 0.7 to 1.3, preferably from 0.85 to 1.15,    particularly preferably form 0.95 to 1.05, very particularly    preferably 1,-   A and B is from 0 to 1 and-   y is a number which is determined by the valency and frequency of    the elements other than oxygen;    Cu₁Mo_(A)W_(B)O_(y)  (VII),    where-   1/(A+C): is from 0.7 to 1.3, preferably from 0.85 to 1.15,    particularly preferably from 0.95 to 1.05, very particularly    preferably 1,-   A and C is from 0 to 1 and-   y is a number which is determined by the valency and frequency of    the elements other than oxygen.

The preparation of such oxometallates is disclosed, for example, in EP-A668 104.

Suitable phases B are also those which contain the oxometallates of thefollowing stoichiometry IXCu₁Mo_(A)W_(B)V_(C)Nb_(D)Ta_(E)O_(y)  (IX),where

-   1/(A+B+C+D+E) is from 0.7 to 1.3, preferably from 0.85 to 1.15,    particularly preferably from 0.95 to 1.05, very particularly    preferably 1,-   (B+C+D+E)/A is from 0 to 1, preferably from 0.05 to 0.3,    particularly preferably from 0.075 to 0.15, very particularly    preferably 0.11, and-   y is a number which is determined by the valency and frequency of    the elements other than oxygen,    and of the structure type defined as the HT copper molybdate    structure, which is characterized by an X-ray diffraction pattern    (fingerprint) whose most characteristic and most intense diffraction    lines, stated as interplanar spacings d [Å], are as follows:    -   6.79±0.3    -   3.56±0.3    -   3.54±0.3    -   3.40±0.3    -   3.04±0.3    -   2.96±0.3    -   2.67±0.2    -   2.66±0.2    -   2.56±0.2    -   2.36±0.2    -   2.35±0.2    -   2.27±0.2    -   2.00±0.2    -   1.87±0.2    -   1.70±0.2    -   1.64±0.2    -   1.59±0.2    -   1.57±0.2    -   1.57±0.2    -   1.55±0.2    -   1.51±0.2    -   1.44±0.2.

Where the phase B contains a mixture of different oxometallates, amixture of oxometallates having the wolframite structure and HT coppermolybdate structure is preferred. The weight ratio of crystalliteshaving the HT copper molybdate structure to crystallites having thewolframite structure may be from 0.01 to 100, from 0.1 to 10, from 0.25to 4 and from 0.5 to 2.

The preparation of oxometallates IX is disclosed, for example, in DE-A195 28 646.

The C phase preferably consists of crystallites which have the trirutilestructure type of α- and/or β-copper antimonate CuSb₂O₆. α-CuSb₂O₆crystallizes in a tetragonal trirutile structure (E.-O. Giere et al., J.Solid State Chem. 131 (1997), 263-274), whereas β-CuSb₂O₆ has amonoclinically distorted trirutile structure (A. Nakua et al., J. SolidState Chem. 91 (1991), 105-112, or comparative diffraction pattern inIndex card 17-284 in the JCPDS-ICDD Index 1989). Moreover, preferred Cphases are those which have the pyrochlore structure of the mineralparzite, a copper antimony oxide hydroxide having the variablecomposition Cu_(y)Sb_(2-x)(O, OH, H₂O)₆₋₇(y≦2.0≦x≦1) (B. Mason et al.,Mineral. Mag. 30 (1953), 100-112, or comparative diagram in Index card7-303 of the JCPDS-ICDD Index 1996).

Furthermore, the C phase may consist of crystallites which have thestructure of the copper antimonate Cu₉Sb₄O₁₉ (S. Shimada et al., Chem.Lett. (1983), 1875-1876 or S. Shimada et. al., Thermochim. Acta 133(1988), 73-77, or comparative diagram in Index card 45-54 of theJCPDS-ICDD Index) and/or the structure of Cu₄SbO_(4,5) (S. Shimada etal., Thermochim. Acta 56 (1982), 73-82 or S. Shimada et al., Thermochim.Acta 133 (1988), 73-77, or comparative diagram in Index card 36-1106 ofthe JCPDS-ICDD Index).

Of course, the regions C may also consist of crystallites which are amixture of the abovementioned structures.

The intimate dry blends on which the multielement oxide active materialsof the formula III are based and which are to be subjected to thethermal treatment according to the invention can be obtained, forexample, as described in WO 02/24327, DE-A 4405514, DE-A 4440891, DE-A19528646, DE-A 19740493, EP-A 756894, DE-A 19815280, DE-A 19815278, EP-A774297, DE-A 19815281, EP-A 668104 and DE-A 19736105. According to theinvention, it is merely necessary to take into account the concomitantincorporation of ammonium ions.

This means that all intimate dry blends produced in the exemplaryembodiments in the abovementioned publications can be subjected to thethermal treatment according to the invention and lead to direct productsof the process which are very useful for catalysts for theheterogeneously catalyzed partial gas-phase oxidation of acrolein toacrylic acid.

The fundamental principle of the preparation of intimate dry blendswhich, in the novel treatment, lead to advantageous multielement oxideactive materials of the formula III comprises preforming at least onemultielement oxide material B (X₁ ⁷Cu_(h)H_(i)O_(y)) as startingmaterial 1 and, if required, one or more multielement oxide materialsC(X₁ ⁸Sb_(j)H_(k)O_(z)) as starting material 2 either separately fromone another or together with one another in finely divided form and thenbringing the starting materials 1 and, if required, 2 into intimatecontact with a mixture which contains sources of the elementalconstituents of the multielement oxide material AMo₁₂V_(a)X_(a)X_(b) ¹X_(c) ²X_(d) ³X_(e) ⁴X_(f) ⁵X_(g) ⁶O_(x)  (A),in a composition corresponding to the stoichiometry A, in the desiredratio (according to the formula III), and, if required, drying theresulting intimate mixture. According to the invention, all that isimportant is that either the sources of the elemental constituents ofthe multielement oxide material A contain ammonium ions and/orcompletely decomposable salts containing ammonium ions, such as NH₄NO₃,NH₄Cl, ammonium acetate, etc., are added when bringing into intimatecontact.

The components of the starting materials 1 and, if required, 2 can bebrought into contact with the mixture (starting material 3) containingthe sources of the elemental constituents of the multimetal oxidematerial A either in dry form or in wet form. In the latter case, it ismerely necessary to ensure that the preformed phases (crystallites) Band, if required, C do not go into solution. In the aqueous medium, thelatter is usually ensured at a pH which does not deviate too greatlyfrom 7 and at temperatures which are not too high. If the bringing intointimate contact is effected in wet form, the intimate dry blend to besubjected to the thermal treatment according to the invention is usuallyfinely dried (for example by spray drying). In the course of dry mixing,such a dry material is automatically obtained. The phases B and, ifrequired, C preformed in finely divided form can of course also beincorporated into a plastically deformable mixture which contains thesources of the elemental constituents of the multimetal oxide materialA, as recommended in DE-A 10046928. Of course, the components of thestarting materials 1 and, if required, 2 can be brought into intimatecontact with the sources of the multielement oxide material A (startingmaterial 3) also as described in DE-A 19815281.

According to the invention, it may also be expedient to adopt aprocedure in which the sources of the multielement oxide material A arethoroughly mixed (for example, dissolved and/or suspended in an aqueousmedium and the aqueous mixture then spray dried) and the resultingfinely divided starting material 3 is mixed with the finely dividedstarting material 1 and, if required 2, and kneaded with one anotherwith addition of water and, if required, other plasticizers. The mixingcan be effected in a special mixing apparatus outside the kneader or inthe kneader itself (change of running direction). The latter isadvantageous. The kneaded material is then extruded and the extrudatesare dried. The extrudates can subsequently be subjected, as described,to the thermal treatment according to the invention. The resultingcalcination material is then usually milled and is used as described inEP-A 714700 for the preparation of coated catalyst.

Suitable sources of the multielement oxide material A are in principleall those which are also suitable as sources for the multielement oxidematerials I.

Suitable plasticizers are substantially all solvents which vaporizesubstantially without residue at up to about 360° C. or decomposesubstantially without residue.

The plasticizer is expediently chosen so that it thoroughly wets thefinely divided dry blend of the starting materials 1, 3 and, ifrequired, 2. Suitable plasticizers in addition to water are carboxylicacids, which may be branched or straight-chain, saturated orunsaturated, e.g. formic acid and acetic acid, primary or secondary C₁-to C₈-alcohols, such as methanol, ethanol or 2-propanol, and alsoaldehydes or ketones, such as acetone, and mixtures thereof.

Kneaders which may be used are, for example, continuous screw kneadersor trough kneaders. Continuous screw kneaders have one or more screwswhich are parallel to the axis and arranged in a cylindrical housing andhave on a shaft kneading and transport elements which convey thematerial added at one end of the kneader to the outlet end of thekneader and at the same time effect plastication and homogenization.Trough kneaders having at least two horizontally mounted rotors, forexample a twin-blade trough kneader having two counterrotatable kneadingblades in a double-depression trough, are expedient in terms ofapplication technology. The rotors may have different shapes, such as asigma, masticator or hub shape, etc. The kneaders may operate batchwiseor continuously. Alternatively, it is also possible to use high-speedintensive mixers, such as plough share mixers or inclined mixers, or acomparatively low-speed Simpson mixer, which, if required, are equippedwith high-speed knife elements.

It is advantageous if the kneading is effected at less than 90° C.,preferably less than 80° C., particularly preferably less than 60° C. Asa rule, the temperature during the kneading is more than 0° C., ingeneral from 20 to 45° C.

It is furthermore advantageous if the kneading takes less than 10,preferably less than 3, hours, particularly preferably less than 1 hour.As a rule, the kneading takes more than 15 minutes.

The plastic (plastic or pasty denotes a consistency which is coherentand does not consist of discrete particles like powder and is deformableonly under the action of a certain force and does not, like a solutionor suspension, readily assume the shape of a vessel) material obtainedafter the kneading can be shaped directly into moldings of any desiredgeometry and, after they have been dried, these moldings can besubjected to the thermal treatment according to the invention, with theresult that unsupported catalysts can be directly obtained. The use of ascrew extruder (alternatively, an Alexander unit or a ram extruder canbe used) is particularly suitable for this purpose. Frequently, theextrudate has a diameter of, for example, from 1 to 20 mm, frequentlyfrom 3 to 10 mm, over a length of, for example, from 0.5 to 20 cm. Asdescribed above, the extrudates can then be dried, thermally treated andmilled and the milled material can be processed to give coatedcatalysts. The drying is carried out, as a rule, at from 50 to 180° C.,in general at from about 120 to 130° C. (expediently in an air stream).

According to the invention, the finely divided starting materials 1, 2generally advantageously consist of particles whose maximum diameter d(longest distance passing through the center of gravity of the particleand connecting two points present on the surface of the particles) isfrom >0 to 300 μm, preferably from 0.01 to 100 μm, particularlypreferably from 0.05 to 20 μm. However, the particle diameter d can ofcourse also be from 0.01 to 150 μm or from 0.5 to 50 μm.

It is possible for the starting materials 1, 2 to be used according tothe invention to have a specific surface area O (determined according toDIN 66131 by gas adsorption (N₂) according to Brunner-Emmert-Teller,(BET)) which is ≦80 m²/g, frequently ≦50 or ≦10 and in some cases ≦1m²/g. As a rule, O>0.1 m²/g.

In principle, according to the invention, the starting materials 1, 2can be used in amorphous and/or crystalline form.

It is advantageous for the starting materials 1, 2 to consist ofcrystallites of oxometallates B (for example those of the formulae V toIX) and crystallites of oxometallates C, as described above. As statedabove, such oxometallates B are obtainable, for example, by theprocedures of EP-A 668 104 or DE-A 19 528 646. However, the preparationprocesses of DE-A 44 05 514 and of DE-A 44 40 891 can also be used.

In principle, multielement oxide materials B containing oxometallates Bor consisting of oxometallates B can be prepared in a simple manner byproducing, from suitable sources of their elemental constituents, a veryintimate, preferably finely divided dry blend having a compositioncorresponding to their stoichiometry, and calcining said dry blend attemperatures (material temperatures) of from 200 to 1000° C., preferablyfrom 250 to 900° C. or from 700 to 850° C., for several hours underinert gas, e.g. nitrogen, or a mixture of inert gas and oxygen orpreferably in the air, it being possible for the duration of calcinationto be from a few minutes to a few hours. The calcination atmosphere mayadditionally contain steam. Calcination under pure oxygen is likewisepossible. Suitable sources of the elemental constituents of themultimetal oxide material B are those compounds which are already oxidesand/or those compounds which can be converted into oxides by heating, atleast in the presence of oxygen. In addition to the oxides, suitablesuch starting compounds are in particular halides, nitrates, formates,oxalates, citrates, acetates, carbonates, ammine complex salts, ammoniumsalts and/or hydroxides (compounds such as NH₄OH, (NH₄)₂CO₃, NH₄NO₃,NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂ or ammonium oxalate, which decompose and/orcan be decomposed at the latest during the subsequent calcination togive compounds escaping in gaseous form, may additionally beincorporated).

The thorough mixing of the starting compounds for the preparation ofmultielement oxide materials B can be effected in dry form or in wetform. If it is effected in dry form, the starting compounds areexpediently used in the form of finely divided powder and, 20: aftermixing and, if required, compaction, are subjected to the calcination.However, the thorough mixing is preferably effected in wet form.Usually, the starting compounds are mixed with one another in the formof an aqueous solution and/or suspension. In the drying methoddescribed, particularly intimate dry blends are obtained whenexclusively dissolved sources of the elemental constituents are used asstarting materials. A preferably used solvent is water. The aqueousmaterial obtained is then dried, the drying process preferably beingeffected by spray drying of the aqueous mixture at outlet temperaturesof from 100 to 150° C. The dryed material is then calcined as describedabove.

A preferred method for the preparation of oxometallates B (X⁷₁Cu_(h)H_(j)O_(y) where X=Mo and/or W) comprises adding an aqueousammoniacal solution of copper carbonate (for example, having thecomposition Cu(OH)₂CO₃) or copper acetate and/or copper formate to anaqueous solution of ammonium heptamolybdate and ammonium paratungstate,drying, e.g. spray drying, the resulting mixture and calcining theresulting dry blend in the manner described, if required aftersubsequent kneading and extrusion as well as drying.

In another variant for the preparation of the multielement oxidematerials B, the thermal treatment of the mixture of the startingcompounds used is effected in a pressure-resistant vessel (autoclave) inthe presence of steam at superatmospheric pressure and at from >100 to600° C. The pressure range is typically up to 500, preferably up to 250,atm. Particularly advantageously, this hydrothermal treatment is carriedout in the temperature range from >100 to 374.15° C. (criticaltemperature of water), in which steam and liquid water coexist under theresulting pressures.

The multielement oxide materials B obtainable as described above, whichmay contain oxometallates B of an individual structure type or a mixtureof oxometallates B of different structure types, or consist exclusivelyof oxometallates B of an individual structure type or of a mixture ofoxometallates of different structure types, can then be used, ifrequired after milling and/or classification to desired sizes, asstarting material 1 required according to the invention.

The multimetal oxide materials C can in principle be prepared in thesame way as the multimetal oxide materials III. In the case of themultimetal oxide material C, the calcination of the intimate dry blendis expediently effected at temperatures (material temperatures) of from250 to 1200° C., preferably from 250 to 850° C. The calcination can becarried out under inert gas, e.g. nitrogen, or under a mixture of inertgas and oxygen, e.g. air, or under pure oxygen. Calcination under areducing atmosphere is also possible. For example, hydrocarbons, such asmethane, aldehydes, such as acrolein or ammonia can be used as suchreducing gases. The calcination can, however, also be carried out undera mixture of O₂ and reducing gases, as described, for example, in DE-A4335973. In a calcination under reducing conditions, however, it must beensured that the metallic constituents are not reduced to the element.Here too, the required duration of calcination (as a rule a few hours)generally decreases with increasing calcination temperature.

According to the invention, preferably used multielement oxide materialsC are those which are obtainable by preparing a dry blend from sourcesof the elemental constituents of the multielement oxide material C whichcontain at least a part, preferably the total amount, of the antimony inthe oxidation state +5, and calcining said dry blend at temperatures(material temperatures) of from 200 to 1200° C., preferably from 200 to850° C., particularly preferably from 300 to 600° C. Such multimetaloxide materials C are obtainable, for example, by the preparationmethods described in detail in DE-A 24 07 677. Preferred among these isthe procedure in which antimony trioxide or Sb₂O₄ is oxidized in anaqueous medium by means of hydrogen peroxide in an amount which is belowthe stoichiometric amount or is equal to it or exceeds it, at from 40 to100° C., to give antimony (V) oxide hydroxide, aqueous solutions and/orsuspensions of suitable starting compounds of the other elementalconstituents of the multimetal oxide material C are added before thisoxidation, during this oxidation and/or after this oxidation, theresulting aqueous mixture is then dried (preferably spray-dried,entrance temperature: from 200 to 300° C., exit temperature: from 80 to130° C., frequently from 105 to 115° C.) and the intimate dry blend isthen calcined in the manner described.

In the process as described above, for example, aqueous hydrogenperoxide solutions having an H₂O₂ content of from 5 to 33% by weight canbe used. Subsequent addition of suitable starting compounds of the otherelemental constituents of the oxometallate C is advisable in particularwhen they are capable of catalytically decomposing the hydrogenperoxide. However, it would of course also be possible to isolate theantimony (V) oxide hydroxide from the aqueous medium and, for example,to mix it thoroughly with suitable finely divided starting compounds ofthe other elemental constituents of the oxometallate C and, if required,further Sb starting compounds and then to calcine this intimate mixturein the manner described.

It is important that the elemental sources of the oxometallates C areeither oxides or are compounds which can be converted into oxides byheating, in the presence or absence of oxygen. In addition to theoxides, particularly suitable starting compounds are therefore halides,nitrates, formates, oxalates, acetates, carbonates and/or hydroxides(compounds such as NH₄OH, (NH₄)₂CO₃, NH₄NO₃, NH₄CHO₂, CH₃COOH, NH₄CH₃CO₂or ammonium oxalate, which decompose and/or can be decomposed at thelatest during the subsequent calcination to give compounds escaping ingaseous form, may additionally be incorporated).

In general, the thorough mixing of the starting compounds in dry or inwet form can also be carried out for the preparation of oxometallates C.If it is effected in dry form, the starting compounds are expedientlyused in the form of finely divided powder. Preferably, however, thethorough mixing is effected in wet form. Usually, the starting compoundsare mixed with one another in the form of an aqueous solution and/orsuspension. After the mixing process is complete, the fluid material isdried and is calcined after drying. Here too, the drying is preferablyeffected by spray drying. After calcination is complete, theoxometallates C can be comminuted again (for example by wet or drymilling, e.g. in a ball mill or by jet milling) and the particle classhaving a maximum particle diameter in the maximum diameter range desiredfor the multielement oxide (III) obtainable according to the inventioncan be separated from the powder obtainable thereby, and as a rulesubstantially comprising spherical particles, by classification (e.g.wet or dry sieving) to be carried out in a manner known per se. Apreferred method of preparation of oxometallates C of the formula (Cu,Zn)₁Sb_(h)H_(i)O_(y) comprises first converting antimony trioxide and/orSb₂O₄ in an aqueous medium by means of hydrogen peroxide into apreferably finely divided, Sb (V) compound, e.g. Sb (V) oxide hydroxidehydrate, adding an ammoniacal aqueous solution of zinc carbonate and/orcopper carbonate (which may have, for example, the compositionCu₂(OH)₂CO₃) or zinc acetate and/or copper acetate and/or zinc formateand/or copper formate to the resulting aqueous suspension, drying theresulting aqueous mixture, for example spray drying it in the mannerdescribed, and calcining the resulting powder, if required aftersubsequent kneading with water and subsequent extrusion and drying, inthe manner described. Under advantageous conditions, the oxometallates Band the oxometallates C can be prepared in a form associated with oneanother. In these cases, a mixture of crystals of the oxometallates Band crystallites of the oxometallates C is obtained, which mixture canbe used directly as starting material 1+2.

For the preparation of an aqueous solution required as starting material3, starting from the abovementioned sources of the elementalconstituents, it is as a rule necessary to use elevated temperatures. Asa rule, temperatures of ≦60° C., in general ≦70° C., but usually ≦100°C., are used. The latter and the following apply in particular whenammonium heptamolybdate tetrahydrate [AHM=(NH₄)₆Mo₇O₂₄.4H₂O] is used asthe Mo element source and/or ammonium metavanadate [AMV=NH₄VO₃] is usedas the vanadium source. The conditions are particularly difficult whenthe element W is a component of the aqueous starting material 3 andammonium paratungstate heptahydrate [APW=(NH₄)₁₀W₁₂O₄₁.7H₂O] is used inaddition to at least one of the two abovementioned elemental sources asa starting compound of the relevant aqueous solution.

It has now surprisingly been found that aqueous solutions prepared atelevated temperatures as starting material 3 are usually stable duringand after the subsequent cooling below the dissolution temperature, evenat elemental Mo contents of ≦10% by weight and cooling temperatures downto 20° C. or less (generally not <0° C.), based on the aqueous solution,i.e. no solid is precipitated during or after the cooling of the aqueoussolution. The above statement applies as a rule also in the case of Mocontents of up to 20% by weight, on the same basis. The same applies forV and W.

Usually, the Mo content of such aqueous solutions cooled to 20° C. orless (generally not below 0° C.) and suitable as starting material 3 isnot more than 35% by weight, based on the solutions.

The above finding is due to the fact that the dissolution at elevatedtemperatures evidently results in compounds of the relevant elementswhich have higher water solubility. This concept is supported by thefact that the residue obtainable from such an aqueous solution by drying(e.g. spray drying) also has a correspondingly higher solubility (evenat the corresponding low temperatures) in water.

An expedient procedure is therefore as follows. At a temperatureT_(L)≧60° C. (for example at up to 65° C. or at up to 75° C. or at up to85° C. or at up to 95° C. or ≦100° C.), an aqueous solution suitable asstarting material 3 is produced. The finely divided solid startingmaterials 1, 2 are then incorporated into this aqueous solution aftercooling to a temperature T_(E)<T_(L). Frequently, T_(L) is ≧70° C. andT_(E) is ≦70° C. If slightly lower dissolution rates and lower solidscontents are accepted, however, T_(L)≦60° C. is also possible.

The incorporation of the prepared solid starting materials 1, 2 into theaqueous starting material 3 (aqueous solution or aqueous suspension ormaterial kneaded to a paste with water) is usually effected by additionof the starting materials 1, 2 to the aqueous starting material 3 cooledas stated above and subsequent mechanical mixing, for example with theuse of stirring or dispersing aids, over a period of from a few minutesto hours, preferably from 20 to 40 minutes. As stated above, it isparticularly advantageous according to the invention if theincorporation of the solid starting materials 1, 2 into the aqueousstarting material 3 is effected at ≦70° C., preferably ≦60° C.,particularly preferably ≦40° C. As a rule, the incorporation temperatureis ≦0° C.

It is furthermore advantageous if the solid starting materials 1, 2 areincorporated into an aqueous starting material 3 whose pH at 25° C. isfrom 4 to 7, preferably from 5 to 6.5. The latter can be achieved, forexample, by adding one or more pH buffer systems to the aqueous startingmaterial 3. For example, the addition of ammonia and acetic acid and/orformic acid or the addition of ammonium acetate and/or ammonium formateis suitable as such buffer systems. Of course, ammonium carbonate mayalso be concomitantly used for the abovementioned purpose. The drying ofthe aqueous mixture obtained on incorporation of the starting materials1, 2 into the aqueous starting material 3 is usually effected by spraydrying. Advantageously, outlet temperatures of from 100 to 150° C. areestablished. As always in this document, spray drying can be effectedeither cocurrently or countercurrently.

Our own investigations have shown that, in the novel thermal treatmentof the intimate dry blend containing the starting materials 1, 3 and, ifrequired, 2, the structure type of the crystallites contained in thestarting materials 1, 2 is substantially retained or at most istransformed into other structure types. However, fusion (dissolution inone another) of the components of the starting materials 1, 2 with oneanother or with those of the starting material 3 substantially does nottake place.

As indicated above, this opens up the possibility of separating off theparticle class having a maximum diameter desired for the multielementoxide material (III) (as a rule from >0 to 300 μm, preferably from 0.01to 100 μm, particularly preferably from 0.05 to 20 μm) by classificationto be carried out in a manner known per se (for example wet or drysieving), after milling of the preformed starting materials 1, 2, andthus of using it tailored-made for the preparation of the desiredmultielement oxide material.

In principle, the multielement oxide materials III obtainable accordingto the invention, and also the multielement oxide active materials 1, IIobtainable according to the invention, can be used in powder form ascatalysts for the heterogeneously catalyzed partial gas-phase oxidationof acrolein to acrylic acid (for example, in a fluidized bed orfluidized bed reactor).

As in the case of the multielement oxide active materials I, II,however, the multielement oxide active materials III are also used,preferably after shaping into moldings having a certain geometry, ascatalysts for the abovementioned partial oxidation. The shaping andchoice of the geometry can be carried out as described above for themultielement oxide active materials I, II.

This means that either the intimate dry blend still to be subjected tothe thermal treatment according to the invention or the multielementoxide active material III itself, obtained according to the inventiontherefrom, can be applied to premolded inert catalyst supports. In theformer case, the novel thermal treatment is carried out after coating ofthe catalyst supports is complete. Application to the catalyst supportis preferably effected after the novel thermal treatment.

The conventional support materials, such as porous or nonporousaluminas, silica, thorium dioxide, zirconium dioxide, silicon carbide orsilicates, such as magnesium silicate or aluminum silicate, can be used.The supports may have a regular or irregular shape, supports having aregular shape and substantial surface roughness, for example spheres orhollow cylinders, being preferred. For example, the use of substantiallynonporous, spherical steatite supports which have a rough surface andwhose diameter is from 1 to 8 mm, preferably from 4 to 5 mm, issuitable. The coat thickness of the active material is expedientlychosen to be in the range from 50 to 500 μm, preferably from 150 to 250μm. However, as described above, hollow cylinders (rings) can also beused as supports. It should be pointed out here that, for the coating ofthe supports in the preparation of such coated catalysts, the powdermaterial to be applied or the support is as a rule moistened with binderand, after the application, is dried again, for example by means of hotair.

For the preparation of the coated catalysts, the coating of the supportsis carried out as a rule in a suitable container, as disclosed, forexample, in DE-A 29 096 71 or in EP-A 293 859. Coating and shaping arepreferably effected as described in EP-A 714700.

The novel process can be applied in a suitable manner in such a way thatthe resulting multielement oxide active materials (III) have a specificsurface area of from 0.50 to 150 m²/g, a specific pore volume of from0.10 to 0.90 cm³/g and a pore diameter distribution such that at least10% of the total pore volume are in each of the diameter ranges from0.1<1 μm, from 1.0 to <10 μm and from 10 to 100 μm. The pore diameterdistributions stated as being preferred in EP-A 293 859 can also beestablished.

Of course, the novel multielement oxide materials (III) can also beoperated as unsupported catalysts. In this respect, the intimate dryblend comprising starting materials 1, 2 and 3 is preferably compacteddirectly to give the desired catalyst geometry (for example by means oftableting or extrusion in a screw extruder or ram extruder), it beingpossible, if required, to add assistants known per se, e.g. graphite orstearic acid as lubricants and/or molding assistants and reinforcingagents, such as microfibers of glass, asbestos, silicon carbide orpotassium titanate, and is subjected to the thermal treatment accordingto the invention. Here too, the thermal treatment according to theinvention can in general be effected before the shaping. Preferredunsupported catalyst geometries are hollow cylinders having an externaldiameter and a length of from 2 to 10 mm or from 3 to 8 mm and a wallthickness of from 1 to 3 mm.

The multielement oxide active materials obtainable according to theinvention are particularly suitable for catalysts having high activityand selectivity (for a specified conversion) for the gas-phase catalyticoxidation of acrolein to acrylic acid. Acrolein which was produced bythe catalytic gas-phase partial oxidation of propene is usually used inthe process. As a rule, the acrolein-containing reaction gases of thispropene oxidation are used without intermediate purification. Usually,the gas-phase catalytic partial oxidation of the acrolein is carried outin tube-bundle reactors as a heterogeneous fixed-bed oxidation. Oxygen,expediently diluted with inert gases (e.g. in the form of air), is usedas an oxidizing agent in a manner known per se. Suitable diluent gasesare, for example, N₂, CO₂, hydrocarbon, recycled reaction exit gasesand/or steam. As a rule, an acrolein:oxygen:steam:inert gas volume ratioof 1:(1 to 3):(0 to 20):(3 to 30), preferably of 1:(1 to 3):(0.5 to10):(7 to 18), is established in the acrolein partial oxidation. Thereaction pressure is generally from 1 to 3 bar and the total spacevelocity is preferably from 1 000 to 3 500 l(S.T.P.) per l per h.Typical multi-tube fixed-bed reactors are described, for example, in.DE-A 2830765, DE-A 2201528 or U.S. Pat. No. 3,147,084. The reactiontemperature is usually chosen so that the acrolein conversion in asingle pass is above 90%, preferably above 98%. In this respect,reaction temperatures of from 230 to 330° C. are usually required.

The multielement oxide active materials according to the invention andthe catalysts containing them are particularly suitable for thehigh-load procedure described in WO 00/53557 and in DE-A 19910508.However, they are also suitable for the processes of DE-A 10313214, DE-A10313213, DE-A 10313211, DE-A 10313208 and 10313209.

In addition to the gas-phase catalytic partial oxidation of acrolein toacrylic acid, the novel products of the process are, however, alsocapable of catalyzing the gas-phase catalytic partial oxidation of otherorganic compounds, in particular other alkanes, alkanols, alkanals,alkenes and alkenols, preferably of 3 to 6 carbon atoms (e.g. propylene,methacrolein, tert-butanol, methyl ether of tert-butanol, isobutene,isobutane or isobutyraldehyde), to give olefinically unsaturatedaldehydes and/or carboxylic acids, and the corresponding nitriles(ammoxidation, especially of propene to acrylonitrile and of isobuteneor tert-butanol to methacrylonitrile). The preparation of acrolein,methacrolein and methacrylic acid may be mentioned by way of example.However, they are also suitable for the oxidative dehydrogenation ofparaffinic or olefinic compounds.

In principle, a wide range of oven types, e.g. tray ovens, rotary tubefurnaces, belt calciners, fluidized-bed ovens or shaft furnaces, aresuitable for carrying out the novel thermal treatment and for carryingout the calcinations in the preparation of the starting materials 1 and2. According to the invention, rotary tube furnaces are preferablysuitable for this purpose.

EXAMPLE 1. General Description of the Rotary Tube Furnace Used for theNovel Thermal Treatment and for the Calcination in the Preparation ofStarting Materials 1 and 2

A schematic diagram of the rotary tube furnace is shown in FIG. 1attached to this document. The reference numerals below relate to thisFIG. 1.

The central element of the rotary tube furnace is the rotary tube (1).It is 4 000 mm long and has an internal diameter of 700 mm. It isproduced from stainless steel 1.4893 and has a wall thickness of 10 mm.

Reciprocating lances which have a height of 5 cm and a length of 23.5 cmare mounted on the inner surface of the rotary tube furnace. They serveprimarily for lifting and thus thoroughly mixing the material to bethermally treated in the rotary tube furnace.

Four reciprocating lances (a quadruple) are mounted in each caseequidistant (in each case 90° spacing) around the circumference at oneand the same height of the rotary tube furnace. Eight such quadruples(spacing of 23.5 cm in each case) are located along the rotary tubefurnace. The reciprocating lances of two adjacent quadruples arearranged staggered relative to one another on the circumference. Noreciprocating lances are present at the beginning and at the end of therotary tube furnace (first and last 23.5 cm).

The rotary tube rotates freely in a right parallelepiped (2) which hasfour equally long, electrically heated (resistance heating) heatingzones which follow one another along the length of the rotary tube andeach of which surrounds the circumference of the rotary tube furnace.Each of the heating zones can heat the corresponding rotary tube sectionto temperatures of from room temperature to 850° C. The maximum heatingpower of each heating zone is 30 kW. The distance between the electricalheating zone and the outer surface of the rotary tube is about 10 cm. Atthe beginning and at the end, the rotary tube projects about 30 cm outof the right parallelepiped.

The speed may be varied from 0 to 3 revolutions per minute. The rotarytube can be rotated both counterclockewise and clockwise. In the case ofclockwise rotation, the material remains in the rotary tube; in the caseof counterclockwise rotation, the material is transported from the feed(3) to the discharge (4). The angle of inclination of the rotary tuberelative to the horizontal may be varied from 0° to 2°. In batchwiseoperation, it is in fact 0°. In continuous operation, the lowest pointof the rotary tube is at the material discharge. Rapid cooling of therotary tube can be effected by switching off the electrical heatingzones and switching on a fan (5). This aspirates ambient air throughholes (6) present in the lower base of the right parallelepiped andtransports said air through three flaps (7) present in the cover andhaving a variable orifice.

The material feed is controlled by means of a rotary vane feeder (masscontrol). As stated above, the material discharge is controlled by meansof the direction of rotation of the rotary tube.

During batchwise operation of the rotary tube, an amount of from 250 to500 kg of material can be thermally treated. It is usually presentexclusively in the heated part of the rotary tube.

From a lance (8) located on the central axis of the rotary tube, a totalof three thermocouples (9) lead vertically into the material atintervals of 800 mm. They permit the determination of the temperature ofthe material. In this document, the temperature of the material isunderstood as meaning the arithmetic mean of the three thermocoupletemperatures. Within the material present in the rotary tube the maximumdeviation of two measured temperatures is, according to the invention,expediently less than 30° C., preferably less than 20° C., particularlypreferably less than 10° C., very particularly preferably less than 5 or3° C.

Gas streams by means of which the calcination atmosphere or in generalthe atmosphere of the thermal treatment of the material is adjustablecan be passed through the rotary tube.

The heater (10) makes it possible to heat the gas stream passed into therotary tube, prior to its entry into the rotary tube, to the desiredtemperature (e.g. to the temperature desired for the material in therotary tube). The maximum power of the heater is 1×50 kW+1×30 kW. Inprinciple, the heater (10) may be, for example, an indirect heatexchanger. Such a heater can in principle also be used as a cooler. As arule, however, it is an electric heater in which the gas stream ispassed over electrically heated metal wires (expediently CSN flowheater, type 97D/80 from C. Schniewindt KG, 58805 Neuerade, Germany).

In principle, the rotary tube apparatus provides the possibility ofpartly or completely circulating the gas stream passed through therotary tube. The circulation pipe required for this purpose is movablyconnected to the rotary tube at the rotary tube inlet and at the rotarytube outlet via ball bearings or via press-fit graphite seals. Theseconnections are flushed with inert gas (e.g. nitrogen) (sealing gas).The two flushing streams (11) supplement the gas stream passed throughthe rotary tube at the inlet into the rotary tube and at the outlet fromthe rotary tube. Expediently, the rotary tube tapers at its beginningand at its end and projects into the entering or departing tube of thecirculation pipe.

Downstream of the outlet of the gas stream passed through the rotarytube is a cyclone (12), for separating off solid particles entrainedwith the gas stream (the centrifugal separator separates off solidparticles suspended in the gas phase, by cooperation of centrifugal andgravitational forces; the centrifugal force of the gas stream rotatingas a vortex accelerates the sedimentation of the suspended particles).

The recycle gas (24) (the gas circulation) is transported by means of arecycle gas compressor (13) (fan) which aspirates in the direction ofthe cyclone and applies pressure in the other direction. Immediatelydownstream of the recycle gas compressor, the gas pressure is as a ruleabove one atmosphere. A recycle gas outlet (recycle gas can bedischarged via a control valve (14)) is located downstream of therecycle gas compressor. An aperture present downstream of the outlet(cross-sectional tapering by about a factor of 3, pressure reducer) (15)facilitates the outlet. The pressure downstream of the rotary tube exitcan be regulated by means of the control valve. This is effected incooperation with a pressure sensor (16) mounted downstream of the rotarytube exit, the exit gas compressor (17) (fan), which aspirates in thedirection of the control valve, the recycle gas compressor (13) and thefresh gas feed. The pressure (directly) downstream of the rotary tubeexit can be adjusted to be, for example, up to +1.0 mbar above and, forexample, up to −1.2 mbar below the external pressure. This means thatthe pressure of the gas stream flowing through the rotary tube can bebelow the ambient pressure of the rotary tube on leaving the rotarytube.

If it is not intended to circulate, even at least proportionately, thegas stream passed through the rotary tube, the connection betweencyclone (12) and recycle gas compressor (13) is closed according to thethree-way valve principle (26) and the gas stream is passed directlyinto the exit gas purification apparatus (23). In this case, theconnection to the exit gas purification apparatus is likewise closedaccording to the three-way valve principle, said connection beingpresent downstream of the recycle gas compressor. If the gas streamsubstantially comprises air, the latter is in this case aspirated (27)via the recycle gas compressor (13). The connection to the cyclone isclosed according to the three-way valve principle. In this case, the gasstream is preferably sucked through the rotary tube so that the internalpressure of the rotary tube is less than the ambient pressure.

During continuous operation of the rotary tube furnace apparatus, thepressure downstream of the rotary tube exit is advantageously adjustedto be −0.2 mbar below the external pressure. During batchwise operationof the rotary tube apparatus, the pressure downstream of the rotary tubeexit is advantageously adjusted to be −0.8 mbar below the externalpressure. The slightly reduced pressure serves the purpose of avoidingcontamination of the ambient air with gas mixture from the rotary tubefurnace.

Sensors (18) which, for example, determine the ammonia content and theoxygen content in the recycle gas are present between the recycle gascompressor and the cyclone. The ammonia sensor preferably operatesaccording to an optical measuring principle (the absorption of light ofcertain wavelength is proportional to the ammonia content of the gas)and is expediently an apparatus of the type MCS 100 from Perkin & Elmer.The oxygen sensor is based on the paramagnetic properties of oxygen andis expediently an oxygen sensor of the type Oxymat MAT SF 7MB1010-2CA01-1AA1-Z from Siemens.

Between the aperture (15) and the heater (10), gases, such as air,nitrogen, ammonia or other gases, can be metered to the actuallyrecirculated recycle gas fraction (19). Frequently, a base load ofnitrogen is metered in (20). Using a separate nitrogen/air splitter(21), it is possible to respond to the measured value of the oxygensensor.

The discharged recycle gas fraction (22) (exit gas) frequently containsgases which are not completely safe, such as NO_(x), acetic acid, NH₃,etc.), and these are therefore usually separated off (23) in an exit gaspurification apparatus.

For this purpose, the exit gas is as a rule first passed via a scrubbercolumn (substantially a column which is free of internals and contains apacking having separation activity upstream of its exit; the exit gasand aqueous spray mist are passed cocurrently and countercurrently (twospray nozzles having opposite spraying directions)).

On arriving from the scrubber column, the exit gas is passed into anapparatus which contains a fine dust filter (as a rule a bundle of tubefilters), from the interior of which the penetrated exit gas is removed.Finally, incineration is effected in a muffle furnace.

The amount of the gas stream which differs from the sealing gas and isfed to the rotary tube is measured and regulated by means of a sensor(28) of the type Model 455 Jr from KURZ Instruments, Inc., Montery (USA)(measuring principle: thermal convective mass flow measurement using anequal-temperature anenometer).

During continuous operation, material and gas phase are passedcountercurrently through the rotary tube furnace.

In connection with this example, nitrogen always means nitrogen having apurity of >99% by volume.

2. Preparation of the Starting Material 1 (Phase B) Having theStoichiometry Cu₁Mo_(0.5)W_(0.5)O₄

98 l of a 25% strength by weight aqueous NH₃ solution were added to 603l of water. 100 kg of copper(II) acetate hydrate (content: 40.0% byweight of CuO) were then dissolved in the aqueous mixture, resulting ina clear, deep blue aqueous solution 1 which contained 3.9% by weight ofCu and was at room temperature.

Independently of the solution 1, 620 l of water were heated to 40° C.27.4 kg of ammonium heptamolybdate tetrahydrate (81.5% by weight ofMoO₃) were dissolved therein in the course of 20 minutes with stirringwhile maintaining the 40° C. Thereafter, 40.4 kg of ammoniumparatungstate heptahydrate (88.9% by weight of WO₃) were added, andafter heating to 90° C. in the course of 45 minutes, were dissolved atthis temperature with stirring. A clear, yellow-orange aqueous solution2 was obtained.

The aqueous solution 1 was then stirred into the solution 2 which was at90° C., the temperature of the total mixture not falling below 80° C.The resulting aqueous suspension was stirred for a further 30 minutes at80° C. It was then spray dried using a spray dryer of the type S-50-N/Rfrom Niro-Atomizer (Copenhagen) (gas inlet temperature: 315° C., gasoutlet temperature: 110° C., cocurrent). The spray-dried powder had aparticle diameter of from 2 to 50 μm.

100 kg of light-green spray-dried powder thus obtained were metered intoa kneader of the type VIU 160 (sigma blades) from AMK (AachenerMisch-und Knetmaschinen Fabrik) and kneaded with addition of 8 l ofwater (residence time: 30 minutes, temperature: from 40 to 50° C.).Thereafter, the kneaded material was emptied into an extruder and shapedinto extrudates (length: 1-10 cm; diameter: 6 mm) by means of theextruder (from Bonnot Company (Ohio), type: G 103-10/D7A-572K (6″extruder W/Packer)). The extrudates were dried on a belt dryer for 1hour at 120° C. (material temperature). The dried extrudates were thensubjected to the thermal treatment (calcination) in the rotary tubefurnace described under 1., as follows:

-   -   the thermal treatment was effected continuously with a material        feed of 50 kg/h of extrudates;    -   the angle of inclination of the rotary tube relative to the        horizontal was 2°;    -   an air stream of 75 m³(S.T.P.)/h was passed through the rotary        tube countercurrently to the material, said air stream being        supplemented by a total of (2×25) 50 m³(S.T.P.)/h of sealing gas        at 25° C.;    -   the pressure downstream of the rotary tube exit was 0.8 mbar        below the external pressure;    -   the rotary tube rotated counterclockwise at 1.5 revolutions per        minute;    -   no recycle gas procedure was used;    -   during the first pass of the extrudates through the rotary tube,        the temperature of the outer surface of the rotary tube was        brought to 340° C. and the air stream was passed at from 20 to        30° C. into the rotary tube;    -   the extrudates were then passed through the rotary tube with the        same throughput and, apart from the following differences, under        the same conditions:        -   the temperature of the rotary tube wall was brought to 790°            C.;        -   the air stream was heated to 400° C. before being passed            into the rotary tube.

The extrudates having a red-brown color were then milled on a biplexcross-flow classifying mill (BQ 500) from Hosokawa-Alpine (Augsburg) toa mean particle diameter of from 3 to 5 μm. The starting material 1 thusobtained had a BET surface area of ≦1 m²/g. The following phases weredetermined by means of X-ray diffraction:

-   -   1. CuMoO₄-III having the wolframite structure;    -   2. HT copper molybdate.

3. Preparation of the Starting Material 2 (Phase C) Having theStoichiometry CuSb₂O₆

52 kg of antimony trioxide (99.9% by weight of Sb₂O₃) were suspended in216 l of water (25° C.) with stirring. The resulting aqueous suspensionwas heated to 80° C. Stirring was then effected for a further 20 minuteswhile maintaining the 80° C. 40 kg of 30% strength by weight aqueoushydrogen peroxide solution were then added in the course of one hour,the 80° C. being maintained. While maintaining this temperature,stirring was effected for a further 1.5 hours. Thereafter, 20 l of waterat 60° C. were added and an aqueous suspension 1 was thus obtained.618.3 kg of an aqueous ammoniacal copper(II) acetate solution (60.8 g ofcopper acetate per kg of solution and 75 l of a 25% strength by weightaqueous ammonia solution in the 618.3 kg of solution) were stirred intosaid suspension 1 at 70° C. Thereafter, heating was effected to 95° C.and stirring was effected at this temperature for a further 30 minutes.50 l of water at 70° C. were then added and the mixture was heated to80° C.

Finally, the aqueous suspension was spray dried (spray dryer of the typeS-50-N/R from Niro-Atomizer (Copenhagen), gas inlet temperature 360° C.,gas outlet temperature 110° C., cocurrent). The spray-dried powder had aparticle diameter of from 2 to 50 μm.

75 kg of spray-dried powder thus obtained were metered into a kneader ofthe type VIU 160 (sigma blades) from AMK (Aachener Misch-undKnetmaschinen Fabrik) and kneaded with addition of 12 l of water(residence time: 30 minutes, temperature from 40 to 50° C.). Thereafter,the kneaded material was emptied into an extruder (same extruder as inthe preparation of phase B) and shaped by means of the extruder to giveextrudates (length 1-10 cm; diameter 6 mm). The extrudates were dried ona belt dryer for 1 hour at 120° C. (material temperature).

250 kg of extrudates thus obtained were subjected to the thermaltreatment (calcination) in the rotary tube furnace described under 1.,as follows:

-   -   the thermal treatment was effected batchwise with an amount of        250 kg of material;    -   the angle of inclination of the rotary tube relative to the        horizontal was ≈0°;    -   the rotary tube rotated clockwise at 1.5 revolutions per minute;    -   a gas stream of 205 m³(S.T.P.)/h was passed through the rotary        tube; at the beginning of the thermal treatment, said gas stream        consisted of 180 m³(S.T.P.)/h of air and 1×25 m³(S.T.P.)/h of N₂        as sealing gas; the gas stream leaving the rotary tube was        supplemented with a further 1×    -   25 m³(S.T.P.)/h of N₂; 22-25% by volume of this total stream        were recycled into the rotary tube and the remainder discharged;        the amount discharged was supplemented by the sealing gas and,        as the residual amount, by fresh air;    -   the gas stream was passed at 25° C. into the rotary tube;    -   the pressure downstream of the rotary tube exit was 0.5 mbar        below external pressure (atmospheric pressure);    -   the temperature in the material was initially increased in the        course of 1.5 hours linearly from 25° C. to 250° C.; the        temperature in the material was then increased in the course of        2 hours linearly from 250° C. to 300° C. and this temperature        was maintained for 2 hours; thereafter, the temperature in the        material was increased linearly in the course of 3 hours from        300° C. to 405° C. and this temperature was then maintained for        2 hours; thereafter, the heating zones were switched off and, by        activating the rapid cooling of the rotary tube by aspirating        air, the temperature inside the material was reduced in the        course of 1 hour to a temperature below 100° C. and finally to        ambient temperature.

The resulting starting material 2 in powder form had a specific BETsurface area of 0.6 m²/g and the composition CuSb₂O₆. The powder X-raypattern of the powder obtained exhibited substantially the reflectionsof CuSb₂O₆ (comparative spectrum 17-0284 of the JCPDS-ICDD Index).

4. Preparation of the Starting Material 3 Having the StoichiometryMo₁₂V_(3.35)W_(1.38)

900 l of water were initially taken in a stirred kettle at 25° C. withstirring. Thereafter, 122.4 kg of ammonium heptamolybdate tetrahydrate(81.5% by weight of MoO₃) were added and heating was effected to 90° C.with stirring. Thereafter, first 22.7 kg of ammonium metavanadate andfinally 20.0 kg of ammonium paratungstate heptahydrate (88.9% by weightof WO₃) were stirred in while maintaining the 90° C. By stirring for 80minutes altogether at 90° C., a clear orange-colored solution wasobtained. This was cooled to 80° C. Thereafter, first 18.8 kg of aceticacid (≈100% strength by weight, glacial acetic acid) and then 24 l of25% strength by weight aqueous ammonia solution were stirred in whilemaintaining the 80° C.

The solution remained clear and orange-colored and was spray dried usinga spray dryer of the type S-50-N/R from Niro-Atomizer (Copenhagen) (gasinlet temperature: 260° C., gas outlet temperature: 110° C., cocurrent).The resulting spray-dried powder formed the starting material 3 and hada particle diameter of from 2 to 50 μm.

5. Preparation of the Dry Material to be Thermally Treated According tothe Invention and Having the Stoichiometry (Mo₁₂V_(3.46)W_(1.39))_(0.87)(CuMo_(0.5)W_(0.5)O₄)_(0.4)(CuSb₂O₆)_(0.4)

In a trough kneader of the type VIU 160 from AMK (Aachener Misch-undKnetmaschinen Fabrik) having two sigma blades, 75 kg of startingmaterial 3, 5.2 l of water and 6.9 kg of acetic acid (100% by weight ofglacial acetic acid) were initially taken and kneaded for 22 minutes.Thereafter, 3.1 kg of starting material 1 and 4.7 kg of startingmaterial 2 were added and were kneaded for a further 8 minutes (T=40 to50° C.).

Thereafter, the kneaded material was emptied into an extruder (sameextruder as in the preparation of phase B) and shaped by means of theextruder to give extrudates (from 1 to 10 cm length, 6 cm diameter).These were dried on a belt dryer for 1 hour at a temperature (materialtemperature) of 120° C.

306 kg of the dried extrudates were then subjected to the thermaltreatment in a rotary tube furnace described under 1., as follows:

-   -   the thermal treatment was effected batchwise with 306 kg of        material;    -   the angle of inclination of the rotary tube relative to the        horizontal was ≈0°;    -   the rotary tube rotated clockwise at 1.5 revolutions per minute;    -   the material temperature was first increased in the course of 2        hours substantially linearly from 25° C. to 100° C.;    -   during this time, 205 m³(S.T.P.)/h of a stream (substantially)        comprising nitrogen was passed through the rotary tube. In the        steady state (after displacement of the air originally        contained), said nitrogen stream had the following composition:        -   110 m³(S.T.P.)/h of base load−nitrogen (20),        -   25 m³(S.T.P.)/h of sealing gas−nitrogen (11) and        -   70 m³(S.T.P.)/h of recirculated gas (19).    -   The nitrogen sealing gas was fed in at 25° C. The mixture of the        other two nitrogen streams was passed in each case into the        rotary tube at the temperature which the material had in each        case in the rotary tube.    -   the material temperature was then increased from 100° C. to        320° C. at a heating rate of 0.7° C./min;    -   a gas stream of 205 m³(S.T.P.)/h which had the following        composition was passed through the rotary tube until a material        temperature of 300° C. was reached:        -   110 m³(S.T.P.)/h consisting of a base load−nitrogen (20) and            gases liberated in the rotary tube,        -   25 m³(S.T.P.)/h of sealing gas−nitrogen (11) and        -   70 m³(S.T.P.)/h of recirculated gas (19).    -   The nitrogen sealing gas was fed in at 25° C. The mixture of the        other two gas streams was passed in each case into the rotary        tube at the temperature which the material in the rotary tube        had in each case.    -   From the time of exceeding the material temperature of 160° C.        until reaching a material temperature of 300° C., 40 mol % of        the amount M^(A) of ammonia liberated altogether in the course        of the entire thermal treatment of the material were liberated        from the material.    -   on reaching the material temperature of 320° C., the oxygen        content of the gas stream fed to the rotary tube was increased        from 0% by volume to 1.5% by volume and maintained over the        subsequent 4 hours.    -   At the same time, the temperature prevailing in the four heating        zones heating the rotary tube was reduced by 5° C. (to 325° C.)        and thus maintained during the subsequent 4 hours.    -   The material temperature passed through a temperature maximum        which was above 325° C. and did not exceed 340° C., before the        material temperature was reduced again to 325° C.    -   The composition of the 205 m³(S.T.P.)/h gas stream passed        through the rotary tube was changed as follows during this        period of 4 hours.        -   95 m³(S.T.P.)/h consisting of base load−nitrogen (20) and            gases liberated in the rotary tube;        -   25 m³(S.T.P.)/h of sealing gas−nitrogen (11);        -   70 m³(S.T.P.)/h of recirculated gas and        -   15 m³(S.T.P.)/h of air via the splitter (21).    -   The nitrogen sealing gas was fed in at 25° C.    -   The mixture of the other gas streams was fed into the rotary        tube in each case at the temperature which the material in the        rotary tube had in each case.    -   From the time of exceeding the material temperature of 300° C.        until 4 hours had elapsed, 55 mol % of the amount M^(A) of        ammonia liberated altogether in the course of the entire thermal        treatment of the material were liberated from the material        (thus, altogether 40 mol %+55 mol %=95 mol % of the amount M^(A)        of ammonia were liberated until 4 hours had elapsed).    -   with the elapse of 4 hours, the temperature of the material was        increased to 400° C. at a heating rate of 0.85° C./min in the        course of about 1.5 hours.    -   This temperature was then maintained for 30 minutes.    -   The composition of the 205 m³(S.T.P.)/h gas stream fed to the        rotary tube was as follows during this time:        -   95 m³(S.T.P.)/h composed of base load−nitrogen (20) and            gases liberated in the rotary tube;        -   15 m³(S.T.P.)/h of air (splitter (21));        -   25 m³(S.T.P.)/h of nitrogen sealing gas (11) and        -   70 m³(S.T.P.)/h of recirculated gas.    -   The nitrogen sealing gas was fed in at 25° C. The mixture of the        other gas streams was passed into the rotary tube in each case        at the temperature which the material in the rotary tube had in        each case.    -   the calcination was terminated by reducing the temperature of        the material; for this purpose, the heating zones were switched        off and the rapid cooling of the rotary tube was switched on by        aspirating air, and the material temperature was reduced to a        temperature below 100° C. in the course of 2 hours and finally        reduced to ambient temperature;    -   on switching off the heating zones, the composition of the 205        m³(S.T.P.)/h gas stream fed to the rotary tube was changed to        the following mixture:        -   110 m³(S.T.P.)/h composed of base load−nitrogen (20) and            gases liberated in the rotary tube;        -   0: m³(S.T.P.)/h of air (splitter (21));        -   25 m³(S.T.P.)/h of nitrogen sealing gas (11) and        -   70 m³(S.T.P.)/h of recirculated gas.    -   The gas stream was fed to the rotary tube at 25° C.    -   during the entire thermal treatment, the pressure (directly)        downstream of the rotary tube exit was 0.2 mbar below the        external pressure.

6. Shaping of the Multimetal Oxide Active Material

-   -   The catalytically active material obtained in 5. was milled by        means of a biplex cross-flow classifying mill (BQ 500) (from        Hosokawa-Alpine Augsburg) to a finely divided powder, 50% of        whose particles passed through a sieve of mesh size from 1 to 10        μm and whose fraction of particles having maximum dimensions        above 50 μm was less than 1%. The specific surface area of the        multimetal oxide active material powder was . . . cm²/g.    -   Annular supports (7 mm external diameter, 3 mm length, 4 mm        internal diameter, steatite of the type C220 from CeramTec        having a surface roughness Rz of 45 μm) were coated by means of        the milled powder as in S1 of EP-B 714700. The binder was an        aqueous solution of 75% by weight of water and 25% by weight of        glycerol.    -   However, in contrast to the abovementioned example S1, the        active material fraction of the resulting coated catalysts was        chosen to be 20% by weight (based on the total weight of support        and active material). The ratio of powder to binder was adjusted        proportionally.    -   FIG. 2 shows the percentage of M^(A) as a function of the        material temperature in ° C. FIG. 3 shows the ammonia        concentration of the atmosphere A in % by volume over the        thermal treatment as a function of the material temperature in °        C.

7. Testing of the Coated Catalysts

-   -   The coated catalysts were tested as follows in a model catalyst        tube around which a salt bath (mixture of potassium nitrate and        sodium nitrate) flowed:    -   Model catalyst tube: V2A stainless steel, 2 mm wall thickness,        26 mm internal diameter, a centered thermal sleeve (for        receiving a thermocouple) of 4 mm external diameter, 1.56 l of        the free model catalyst tube space were filled with the coated        catalyst.    -   The reaction gas mixture had the following starting composition:    -   4.8% by volume of acrolein, 7% by volume of oxygen, 10% by        volume of steam and 76% by volume of nitrogen, the remaining        amount comprising a mixture of oxides of carbon and oxygenates        of propylene.    -   The model catalyst tube was loaded with 2 800 l(S.T.P.)/h of        reaction starting mixture. The temperature of the salt bath was        adjusted so that an acrolein conversion of 99.3 mol % resulted        during a single pass.    -   The salt bath temperature T required in this respect was 262° C.        and the selectivity of the acrylic acid formation was 96.4 mol %        of acrylic acid.

At this point, it should also be stated that, with these coatedcatalysts, regular loading of tube-bundle reactors having from 5000 to40 000 catalyst tubes (wall thickness typically from 1 to 3 mm, internaldiameter as a rule from 20 to 30 mm, frequently from 21 to 26 mm, lengthtypically from 2 to 4 m) is possible, which loading is such that, owingto the homogeneous preparation of the active material, in a randomsample of 12 catalyst tubes the difference between the arithmetic meanactivity and the highest or lowest activity is not more than 8° C.,frequently not more than 6° C., often not more than 4° C. and inadvantageous cases not more than 2° C.

A measure used for the activity of the catalyst tube loading is thetemperature which a salt bath (mixture of 53% by weight of potassiumnitrate, 40% by weight of sodium nitrite and 7% by weight of sodiumnitrate) flowing around the individual catalyst tube must have in orderthat, in a single pass of reaction gas mixture comprising 4.8% by volumeof acrolein, 7% by volume of oxygen, 10% by volume of steam and 78.2% byvolume of nitrogen (at a space velocity of the catalyst load of 85 l(S.T.P.) of acrolein/l of catalyst load·h) through the loaded catalysttube, an acrolein conversion of 97 mol % is achieved (l of catalyst loaddoes not include the volumes inside the catalyst tube where purepreliminary or subsequent beds of inert material are present, only thebed volumes which contain catalyst moldings (if required, diluted withinert material).

Such catalyst load are particularly suitable for higher acroleinvelocities (e.g. ≧135 l to 350 l (S.T.P.) per l per h or more).

Comparative Example 1

Everything was carried out as in Example 1. However, the stream of gasmixture fed to the rotary tube contained from the outset (i.e. on theway to the material temperature of 100° C. or 300° C.) 95 m³(S.T.P.)/hcomposed of base load of nitrogen (20) and gases liberated in the rotarytube and 70 m³(S.T.P.)/h of recirculated gas, but from the outset 15m³(S.T.P.)/h of air (splitter (21)) and thus 1.5% by volume of molecularoxygen.

The salt bath temperature required in the testing of the coated catalystfor an acrolein conversion of 99.3 mol % was 268° C. and the selectivityof the acrylic acid formation was only 94.2 mol %.

Comparative Example 2

Everything was carried out as in Example 1. During the entire durationof the thermal treatment, however, the gas stream fed to the rotary tubecontained no air at all (instead of the air stream according to theexample, the corresponding nitrogen stream was always fed via thesplitter).

The salt bath temperature required in the testing of the coated catalystfor an acrolein conversion of 99.3 mol % was 279° C. and the selectivityof the acrylic acid formation was only 91.0 mol %.

Example 2

Everything was carried out as in Example 1. The shaping of themultimetal oxide active material was effected, however, as follows:

70 kg of annular supports (7.1 mm external diameter, 3.2 mm length, 4.0mm internal diameter, steatite of type C220 from CeramTec, having asurface roughness R_(z) of 45 μm and a total pore volume, based on thevolume of the support, of ≦1% by volume) were introduced into a coatingpan (angle of inclination 90°; Hicoater from Lödige, Germany) having aninternal volume of 200 l. The coating pan was then caused to rotate at16 rpm. From 3.8 to 4.2 liters of an aqueous solution of 75% by weightof water and 25% by weight of glycerol were sprayed onto the supportsvia a nozzle in the course of 25 minutes. At the same time, 18.1 kg ofthe milled multimetal oxide active material were continuously meteredvia a vibrating channel outside the spray cone of the atomizer nozzle inthe same period. During the coating, the powder fed in was completelytaken up onto the surface of the support, and agglomeration of thefinely divided oxidic active material was not observed. After the end ofthe addition of active material powder and water, hot air (about 400m³/h) was blown into the coating pan at a rotation speed of 2 rpm for 40minutes (alternatively from 15 to 60 minutes) at 100° C. (alternativelyfrom 80 to 120° C.). Annular coated catalysts whose proportion ofanoxidic active material was 20% by weight, based on the total material,were obtained. The coat thickness was 170±50 μm, considered both overthe surface of one support and over the surface of different supports.

The testing of the coated catalysts was effected as in Example 1. Theresults obtained corresponded to the results achieved in Example 1.

FIG. 4 also shows the pore distribution of the milled active materialpowder before its shaping (its specific surface area was 21 m²/g). Thepore diameter in μm is plotted along the abscissa (logarithmic scale).

The logarithm of the differential contribution in ml/g of the respectivepore diameter to the total pore volume is plotted along the rightordinate (curve O). The maximum has the pore diameter with the greatestcontribution to the total pore volume. The integral over the individualcontributions of the individual pore diameters to the total pore volumeis plotted along the left ordinate, in ml/g (curve □). The end point isthe total pore volume (all data in this document on determinations oftotal pore volumes and of diameter distributions over these total porevolume are, unless stated otherwise, based on determinations by themercury porosimetry method using the Auto Pore 9220 apparatus fromMicromeritics GmbH, 4040 Neuβ, Germany (bandwidth 30 Å to 0.3 mm); alldata in this document on determinations of specific surface areas or ofmicropore volumes are based on determinations according to DIN 66131(determination of the specific surface area of solids by gas adsorption(N₂) according to Brunauer-Emmet-Teller (BET)).

FIG. 5 shows the individual contributions of the individual porediameters (abscissa, in Angström, logarithmic scale) in the microporeregion to the total pore volume for the active material powder beforeits shaping, in ml/g (ordinate).

FIG. 6 shows the same as FIG. 4, but for multimetal oxide activematerial (its specific surface area was 24.8 m²/g) subsequently detachedfrom the annular coated catalyst by scratching off mechanically.

FIG. 7 shows the same as FIG. 5, but for multimetal oxide activematerial subsequently detached from the annular coated catalyst byscratching off mechanically.

Example 3

Everything was carried out as in Example 1. The shaping of themultimetal oxide active material was effected, however, as follows:

70 kg of spherical support (diameter 4 to 5 mm; steatite of type C220from CeramTec, having a surface roughness R_(z), of 45 μm and a totalpore volume, based on the volume of the support, of ≦1% by volume) wereintroduced into a coating pan (angle of inclination 90°; Hicoater fromLödige, Germany) having an internal volume of 200 l. The coating pan wasthen caused to rotate at 16 rpm. From 2.8 to 3.3 liters of water weresprayed onto the support via a nozzle in the course of 25 minutes. Atthe same time, 14.8 kg of the milled multimetal oxide active materialwere continuously metered via a vibrating channel outside the spray coneof the atomizer nozzle in the same period. During the coating, thepowder fed in was completely taken up onto the surface of the supports,and agglomeration of the finely divided oxidic active material was notobserved. After the end of the addition of powder and water, hot air(about 400 m³/h) was blown into the coating pan at a rotational speed of2 rpm for 40 minutes (alternatively from 15 to 60 minutes) at 100° C.(alternatively from 80 to 120° C.). Spherical coated catalysts whoseproportion of oxidic active material was 17% by weight, based on thetotal material, were obtained. The coat thickness was 160±50 μm,considered both over the surface of one support and over the surface ofdifferent supports.

FIG. 8 shows the analog of FIG. 6 (the specific surface area of thescratched-off multimetal oxide active material was 20.3 m²/g).

FIG. 9 shows the analog of FIG. 7.

The testing of the spherical coated catalyst was effected as describedin Example 1 for the annular coated catalyst.

All coated catalysts prepared in this document by way of example areparticularly suitable for acrolein partial oxidation at high acroleinspin velocities of the catalyst load (e.g. ≧135 to 350 l (S.T.P.) per lper h).

1. A process for the preparation of a catalytically active multimetaloxide comprising material which comprises at least one of the elementsNb and W, and the elements Mo, V and Cu, wherein the molar fraction ofthe element Mo, based on the total amount of all elements other thanoxygen in the catalytically active multimetal oxide comprising material,is from 20 to 80 mol %, wherein the molar ratio of Mo to V, Mo/V, isfrom 15:1 to 1:1, wherein the corresponding molar ratio Mo/Cu is from30:1 to 1:3 and the corresponding molar ratio Mo/(total amount of W andNb) is from 80:1 to 1:4, the process comprising preparing an intimatedry blend comprising ammonium ions from starting compounds that comprisethe elemental constituents of the multimetal oxide comprising material,other than oxygen, as components; and thermally treating the intimatedry blend at elevated temperatures in an atmosphere A having a lowcontent of molecular oxygen, so that at least a portion of the ammoniumions contained in the intimate dry blend are decomposed at ≧160° C. withliberation of ammonia, wherein the thermal treatment is carried out asfollows: heating the intimate dry blend at a heating rate of ≦10° C./minto a decomposition temperature in a decomposition temperature range offrom 240° C. to 360° C., and then keeping the temperature in thedecomposition range until at least 90 mol % of the total amount M^(A) ofammonia liberated altogether in the entire course of the thermaltreatment of the intimate dry blend from the intimate dry blend at above160° C. has been liberated; reducing, to less than or equal to 0.5% byvolume, the content of molecular oxygen in the atmosphere A in which thethermal treatment of the intimate dry blend takes place no later thanwhen the intimate dry blend has reached 230° C., and maintaining thereduced oxygen content until at least 20 mol % of the total amount M^(A)of ammonia liberated altogether in the entire course of the thermaltreatment has been liberated; taking the intimate blend out of thedecomposition temperature range, at a rate of ≦10° C./min, and into acalcination temperature range of from 380 to 450° C. no earlier thanwhen ≧70 mol % of the total amount of M^(A) of ammonia liberatedaltogether in the entire course of the thermal treatment has beenliberated increasing the content of molecular oxygen in the atmosphere Ato >0.5 to 4% by volume no later than when 98 mol % of the total amountM^(A) of ammonia liberated altogether in the entire course of thethermal treatment has been liberated and calcining the intimate dryblend at this increased oxygen content of the atmosphere A in thecalcination temperature range.
 2. The process as claimed in claim 1,wherein the content of ammonium ions in the intimate dry blend, prior tothe thermally treating step, is ≧5 mol %, based on the total molarcontent of elemental constituents of the subsequent catalytically activemultimetal oxide comprising material, other than oxygen, in the intimatedry blend.
 3. The process as claimed in claim 1 or 2, wherein thetemperature rate at which the intimate dry blend is heated to thedecomposition temperature is ≦5° C./min.
 4. The process of claim 1,wherein the temperature rate at which the intimate dry blend is heatedto the decomposition temperature is ≦3° C./min.
 5. The process of claim1, wherein the decomposition temperature range is from 300 to 350° C. 6.The process of claim 1, wherein the intimate dry blend is kept in thedecomposition temperature range until at least 95 mol % of the totalamount M^(A) of ammonia liberated altogether in the entire course of thethermal treatment of the intimate dry blend from the intimate dry blendat above 160° C. has been liberated.
 7. The process of claim 1, whereinthe content of molecular oxygen in the atmosphere A is ≦0.3% by volumeno later than when the intimate dry blend has reached 230° C.
 8. Theprocess of claim 1, wherein the content of molecular oxygen in theatmosphere A is increased to >0.5 to 4% by volume before 80 mol % of thetotal amount M^(A) of ammonia liberated altogether in the entire courseof the thermal treatment has been liberated.
 9. The process of claim 1,wherein the content of molecular oxygen in the atmosphere A is increasedto 0.6 to 4% by volume no later than when 98 mol % of the total amountM^(A) of ammonia liberated altogether in the entire course of thethermal treatment has been liberated.
 10. The process of claim 1,wherein the content of molecular oxygen in the atmosphere A is increasedto 1 to 3% by volume no later than when 98 mol % of the total amountM^(A) of ammonia liberated altogether in the entire course of thethermal treatment has been liberated.
 11. The process of claim 1,wherein the calcination temperature range is from 380 to 430° C.
 12. Theprocess of claim 1, wherein, after calcination is complete, the intimatedry blend is cooled to ≦100° C. in the course of a period of ≦5 hours.13. The process as claimed in claim 12, wherein the cooling is effectedto at least 350° C. in an atmosphere A whose content of molecular oxygenis ≦5% by volume.
 14. The process of claim 1, wherein the ammoniacontent of the atmosphere A passes through a maximum which is ≦10% byvolume in the course of the thermal treatment.
 15. The process asclaimed in claim 14, wherein the atmosphere A reaches its maximumammonia content before the intimate dry blend has reached thecalcination temperature range.
 16. The process of claim 1, wherein thecatalytically active multimetal oxide comprising material satisfies thestoichiometry IMo₁₂V_(a)X¹ _(b)X² _(c)X³ _(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(n)  (I), wherein:X¹ is W, Nb, Ta, Cr and/or Ce, X² is Cu, Ni, Co, Fe, Mn and/or Zn, X³ isSb and/or Bi, X⁴ is one or more alkali metals, X⁵ is one or morealkaline earth metals, X⁶ is Si, Al, Ti and/or Zr, a is from 1 to 6, bis from 0.2 to 4, c is from 0.5 to 18, d is from 0 to 40, e is from 0 to2, f is from 0 to 4, g is from 0 to 40 and n is a number which isdetermined by the valency and frequency of the elements other thanoxygen in I.
 17. The process of claim 1, wherein the catalyticallyactive multimetal oxide comprising material is one of the formula III[A]_(p)[B]_(q)[C]_(r) (III) wherein: A is Mo₁₂V_(a)X¹ _(b)X² _(c)X³_(d)X⁴ _(e)X⁵ _(f)X⁶ _(g)O_(x), B is X⁷ ₁Cu_(h)H_(i)O_(y) C is X⁸₁Sb_(j)H_(k)O_(z), X¹ is W, Nb, Ta, Cr and/or Ce, X² is Cu, Ni, Co, Fe,Mn and/or Zn, X³ is Sb and/or Bi, X⁴ is Li, Na, K, Rb, Cs and/or H, X⁵is Mg, Ca, Sr and/or Ba, X⁶ is Si, Al, Ti and/or Zr, X⁷ is Mo, W, V Nband/or Ta, X⁸ is Cu, Ni, Zn, Co, Fe, Cd, Mn, Mg, Co, Sr and/or Ba, a isfrom 1 to 8, b is from 0.2 to 5, c is from 0 to 23, d is from 0 to 50, eis from 0 to 2, f is from 0 to 5, g is from 0 to 50, h is from 0.3 to2.5 i is from 0 to 2, j is from 0.1 to 50, k is from 0 to 50, x, y and zare numbers which are determined by the valency and frequency of theelements other than oxygen in A, B and C, p and q are positive numbersand r is 0 or a positive number, the ratio p/(q+r) being from 20:1 to1:20 and, where r is a positive number, the ratio q/r being from 20:1 to1:20, which contains the moiety [A]_(p) in the form of three-dimensionalregions A having the chemical composition A: Mo₁₂V_(a)X¹ _(b)X² _(c)X³_(d)X⁴ _(e)X⁴ _(f)X⁶ _(g)O_(x), the moiety [B]_(q) in the form ofthree-dimensional regions B having the chemical composition B: X⁷₁Cu_(h)H_(i)O_(y) and the optional moiety [C]_(r) in the form ofthree-dimensional regions C having the chemical composition C: X⁸₁Sb_(j)H_(k)O_(z), the regions A, B and, if desired, C being distributedrelative to one another as in a mixture of finely divided A, finelydivided B and, if desired, finely divided C.
 18. The process of claim 1,wherein the thermal treatment is carried out in a rotary tube furnacethrough which a gas stream flows.
 19. The process as claimed in claim18, wherein the rotary tube furnace is operated batchwise.
 20. Theprocess as claimed in claim 18 or 19, wherein at least a portion of thegas stream running through the rotary tube is circulated.
 21. Theprocess as claimed in claim 18 or 19, wherein the pressure of the gasstream flowing through the rotary tube on leaving the rotary tube isbelow the ambient pressure of the rotary tube.